Impression count changes by the different request date for ad performance report api

[yh...@newsjel.ly]

 We run a site called magictbl.com, pulling data from google Adwords report API. In December, we started including the result from Google display network&Youtube advertisement. We derive yesterday’s impression data in every morning time in Korean standard time.
However, last Friday, we discovered that ap performance report API returns different data when we request this data after a week.
 For instance, for ad A data January 19:
impression was 254 in January 20, was 255 in January 26.
For ad B in January 20 in January 26.
impression was 2249 in January 20, was changed as 2250 in January 26.

We assume that adwords takes a couple of days to finish collecting data from its engine. But, we need clear explanation.
Any idea or comment for this fact? Why the API returns different value after one day and few days? Should we put off the request time after few hours?

[Zweitze]

Sometimes Google applies corrections for invalid traffic. This is especially the case if you advertise on the display network.
Such corrections are typically applied after a few days.

See also the docs on data freshness, especially section "Adjustments".

[Sreelakshmi Sasidharan (AdWords API Team)]

Hi, 

As Zweitze pointed out, the stats could get updated at a later date. Since you are running the stats for the previous day, we recommend re-running the report at a later date to gather any adjustments. You could also rerun them at say end of every week or end of every month and update the stats for those which had any adjustments during that period. 

Thanks,
Sreelakshmi, AdWords API Team