As I understand it the reasoning given tends to be that ad auctions happen extremely quickly, and therefore too quickly to check some single source of truth for your current budget.

Therefore, the budget is only "eventually consistent" - i.e. at the end of the campaign or after a day or so, the number you're being charged is accurate. However during the campaign itself it's not possible to guarantee that things won't go slightly over budget as each individual ad auction cannot feasibly check the central budget.

That said, it definitely feels like there should be a way to implement this such that it backs off as the budget is approached, so that overshoots are likely to be minor, rather than 100% of the budget as apparently happens on a regular basis.

I'm pretty sure fixing bugs that make money is on the very bottom of the list of priorities....

They don't need a bug fix as a stopgap. The fair thing to do would be refund the excess. If I set a limit and they display ads and exceed it, why should it be my problem?

Excess is calculated at end of month not daily

Back when I was in the ads org, there was no month without a big important future launch, that was supposed to make us make less money in the short term.

I suspect a factor that makes this trickier is cost per click. You're bidding for a click not a particular spend, so if your cost per click is ~$0.01 then overrunning by 100 clicks only costs $1 over budget. If you're more like $1-5 a click (not uncommon) then you can easily get to $100 over budget.

My guess is that this is worse the lower your budget, and $100 a day is a low budget for Google ads when you're considering the whole industry.

I would expect that with a budget of, say, $5000 a day, you might still be ~$100 out.

No. They overshoot because there’s a small-letter asterisk within the budged definition that allows then to overshoot at twice the amount.

I wouldn't use the word likely. It might be because of who your target group is etc and I therefore have another experience. But yes, it does happen. The way it works is that Google allow themself to spend up to 2x your daily budget. If they exceed your daily x2, you'll be credited any overcharged amount.

Eg. Your daily budget is $200 and they spend $420, you’ll get a $20 credit.

Then there is the monthly limit, which is your daily budget x 30.4. Also here you'll be credited any amount they exceed.

So basically it sounds like the "budget" is actually more of a "target spend", that they will try to approximately hit in the short run, and hit very accurately on average over a month.

what's the point of that? knowing that, why wouldn't i just set the budget to $50 and avoid the problem that way?

Most likely the algorithms that chooses whose ad gets shown are optimized to not have to look up each persons spend for that day, as syncing that between all servers on every page request would be more expensive than all other steps combined.

I'm not sure how they could let you overrun by several times your budget for a day however, syncing these every hour shouldn't be a problem.

I have explained it in other comment above. Its not technical issue. Its by design

Google explains that on the long run, like,30 daya of campaign, it will be proper on average. The explanationis, that Google will spend more when it sees possibilities to give more value in specific time. For example, there is a special day when people buy more than usual, but later they dont. So there is more budget spend on this special day, and less on others to make it even.

So there is a reason behind it. But its something well hidden. You have 500 dollars for a one week campaign, and it occurs you pay 1000 dollars for it. It does not seem to be right.

There is no reason to that aside from Google making more money. They are not your friend.

Actually yes. Google wants to have advertisers who spend fixed amount each month without going much into details. They dont care much about anything more

For example keyword targetting gets broader.and broader. If you target.for keyword you want to target it precisely or with synonyms but you want to have control over it. But Google changed it lately to make it harder to narrow down keywords.

The result is you spend budget on bad keywords. And it seems like ok,if i get money from it overall.

But actually not only you loose budget but others too, because they need to bid higher.

They need.to bid higher for keywords you dont care about but they care. For Google its additional money.

It's not a bug per se and is designed this way. The ad budgets are calculated on 30 days basis. So if you set the budget to $100 per day, Google will display ads with monthly budget of 100*30 or $3000. The objective is to suck out as much money as possible from the advertiser very quickly.

The only assurance is that your monthly budget won't exceed $3000