I agree... some of my Ancestry results say I'm "Eastern European" when the furthest east in Europe my kin is from is the eastern coast of Italy.
I've also taken the DNA test offered by National Geographic. As far as they're concerned, I'm British and "Mediterranean" (as opposed to Eastern European).
It is really all a matter of the population they have to compare to. Like for NatGeo, my results were extremely similar to present-day Brits and Italians who have also submitted their DNA to NatGeo's genegraphic project, so I'm British and "Mediterranean".
NatGeo has over 500,000 test results.
Whereas Ancestry only has those of who are members and who took the test, maybe a few thousand sets of results right now at best. Over time, no doubt, all of us will find our percentages of ethnicity change.
It would certainly help if Ancestry did a better job of explaining how they come up with percentages, but one thing is sure, whoever you test with, you're just being compared to others who took that exact same test,so your "ethnicity" will appear to change from vendor to vendor (!)