Varying resolution over different countries?

Just wondering...Seems like certain countries/regions the imagery is always of lower resolution, atmospheric conditions not withstanding. Much of the coverage over U.S. and Europe is crystal clear approaching less than 1 meter resolution. Notably coverage over Russia, China, and the Middle East is frequently grainy and rarely hits better than a few meters res.

You can find an article HERE explaining imagery, from GEB.

Thanks for the reference, Ledge. Informative article, but it doesn't address my primary question on varying resolution. Guess I'm wondering if Google (or Digital Globe) is restricted from collecting certain resolution levels by some national governments. Attaching couple of examples; Washington Dulles on the left and Moscow Scheremtyevo on the right. DG imagery over Russia is consistently lower res than imagery over the U.S., or Europe.