Don't forget that these numbers can also be skewed downard by people who only played for a while and never picked up the game again (renters..etc)
I recall Valve releasing the amount of time people played HL2:EP2 and the numbers were shocking, I think like only 30% of people actually completed the game and something like 10% played only 5 minutes or less (booted up the game to see it was working and never played it again)