The fact a game is better than its sequel doesn't mean it'll score higher. Times change, and concepts and gameplay formulas that were once considered great might become stale. For instance, Pokémon, to which GF is always adding new stuff and reworking old concepts just to keep more or less the same reception they got in the 90s, even though the games are now considerably more immersive and rewarding.
Look at gameplay of any game of the Gears of War series and you'll be hard pressed to tell which is which unless you're really into it. Now compare to something like Mario or even another TPS like Mass Effect. Boom, instantly different gameplay, different interface, art direction etc. etc. Gears of War, though? Since 2006 I'm still going to press a button at the right time to reload my gun faster...
