What would cause this?
venrooy, you didn't mention what costs you were referring to - but I think there is a good chance that wedouglas in on the right track.
Adwords randomly change the date on me and gives me a heart attack when I see a full days worth of costs at 9 am haha.
Actually, I have never known this to randomly change. Date range is actually a "sticky" setting that'll stay exactly where you put it till you change it again.
So you may very well see different settings from one session to the next, but what date range you will see when you log in depends on where it was set when you last logged out.
If that isn't the way it's working, I'm sure the tech folks would love to look at your account. ;)
AWA