Adwords maintains a historical record of your account as well as everyone else's account. This history is used to determine where your advert will be positioned on the search results in conjunction with your cost per click (CPC).
Simple rule of thumb:
Bad history = High CPC
Good history = Lower CPC
Whenever new changes are made to an Adwords advert, the keyword/advert couplet history will be ‘reset’. This means actual CPC’s and ad rankings are subject to change – usually in a negative fashion. In other words – if you make a change to an existing adgroup advert, it will make things worse initially.
This is something that the majority of Adwords users do not know about and are perplexed until they realise that their experimentation – without knowledge of the ramifications – caused their increased cost. And this is also why, you should augment existing adgroups rather than edit existing adverts. It is very rare to have ‘instant’ changes create wildly profitable returns – this is why account management is an iterative and evolutionary process.