It’s impossible to give guarantees in an industry where we bid on an auction-based system that fluctuates placement over time.
You can compare it to guaranteeing a friend that he can buy an art piece for $1000 in an auction – You can’t do that.
Even if you have insights to the auction and vast experience as an auctioneer it is impossible to know if that day will bring double the amount of bidders. AdWords works the same way; competition can increase from one day to another.
Furthermore, there are factors that we cannot control. For instance, if your page becomes a lot slower it penalizes your quality score, then our guarantee is worth nothing.
Even Google Warns About AdWords Guarantees
Google warns about agencies that give guarantees for placements or other work in Google AdWords in the official Google AdWords Third Party Partner Disclosure.