Few months back Google increased the overdelivery limit for adwords ads to 100%. Many advertisers have started complaining that on odd days spend goes too far away from what they envisaged.
There is a Daily Budget feature in every campaign. While setting up daily budget you actually tell Google How much you want to spend on any day. However, web traffic does not remain static over time, rather it varies widely across your chosen keyword/topic/interest... candidates. To comply with it Google used to allow your ad to go past your daily budget, so that it may survive the occasional surge in traffic. Earlier it used to be only 20%. Now this is upto 100%, which means Google can spend upto Rs200, if your Campaign Daily Budget is Rs100.
Impact on Advertisers
Advertisers need not panic. Google while delivering more than your daily budget also takes into account the monthly limit, which is Daily Budget x 30.4. So, Google's system while giving you extra exposure on some day always calculates, the deficit and gaps in scheduling to make up for it. That is to say overdelivery algorithm is designed to keep monthly spend within the limit of 30.4*DB and if by any chance it goes past it, Google won't bill you for that. Those clicks are free! So, if you are using ad scheduling limiting exposure of your ads Or to say your daily budget doesn't get consumed because of time limits you have set up or opting out of some days of the week or you are a casual advertiser who turns his campaign on and off manually, you are more likely to see your daily budget getting breached, on odd days. For advertisers, who keep their ads turned on for most of the time, Google won't find gap within Monthly Limit and real monthly spend & so this behavior is unlikely.
Advertisers should calculate 30.4 x Daily Budget, figure and actual expected spend, like if you don't run ads on sundays then you are expecting a total spend of only 26 x DB. Say, your DB is Rs100 & you don't advertise on Sundays & expecting a total spend of Rs2600 only. However, Google will see the limit as 30.4 x 100 = Rs 3040. To not get piqued at the end of the month, just equate the two, 2600 = DB x 30.4, So, DB = 2600/30.4 = Rs85.53 . Hence, if you set up Daily Budget as Rs85.53 with 4 Sundays in a month, you will see effectively a spend of Roughly Rs100 everyday. Happy!