Question About : Third Party Grading Population Reports?

Lately I've been wondering about how accurate the annual Population Reports really are for T.P.G.'s. I know many collectors and dealers that crack open older holders ie:green label P.C.G.S holders and I.C.C.S. holders with the seal on the Certificate.
These coins are re-submitted and I've heard that 90% of the time a higher grade is obtained.
If this being true, there virtually is no credibility in today's Population Reports of the major T.P.G.'s, is there? I've heard stories from good sources of major coin dealers re-submitting key date coins to a T.P.G. companies several times until the desired grade is achieved. It sometimes makes me wonder when I read the Population Reports and see a particular key date coin has X amount of mintstate examples in existence which is stated on the Population Report. However, one seldom ever sees this particular coin on E-bay or in auctions. I'm not referring to any particular key date piece, but in general. I've experienced searching for many scarce coins over the years which the major T.P.G.'s have listed several dozen examples to be known, but none to be found.
Could this be as a result of a coin's numerous re-submissions, perhaps being cracked out and being sent to another T.P.G. in hopes of more liberated grading attitudes?
If you have any opinion or foresight on the thread I've sparked, fill me in!

Glenn