Hello MG,
I've tried running the report query as you suggested using last 30 days as a predicate.
However, I couldn't reproduce the issue you reported.
What I can see is the total number of impressions returned by both CAMPAIGN_PERFORMANCE_REPORT and ACCOUNT_PERFORMANCE_REPORT matches exactly with that shown in UI.
(I can share the data and screenshot with you if you wish)
Did you see the large differences between all these three sources?
During my test, the numbers did change a bit (tens - hundreds in impressions), which is probably because the system was updating some impression data.
So, if you see discrepancy in such a degree, I think it works as intended.
Cheers,
Thanet, AdWords API Team