Valliance, I think you're missing some variables here. You keep saying it's fanboyism and bias, but really, there are other, simpler, and less insulting explanations.
1. We are likely to have more Forza owners here, so there are more potential votes for the franchise. Most people voting in the poll played only a small proportion of the games. Of those small selection of games, it's likely that Xbox owners, especially at a rather hardcore site like this, would choose Xbox games over others (to buy and play, I mean). So what you end up with is higher "base rates" of Forza owners in the poll, therefore more Forza votes than in a more balanced group.
2. Xbox owners themselves are people who are likely to prefer Xbox exclusives. That's why they bought the console in the first place, after all. So Union members are people who are already inclined to prefer games like Forza, Halo, Gears, etc. It would be the same at a Sony or Nintendo leaning site. You'd expect people there to naturally prefer and vote for those types of games -- that's why they bought the console in the first place. That's why they're at the site.
So you've got a dual effect here going on -- 1) base rates of ownership of the game are higher, meaning more potential votes, and 2) Xbox owners themselves are self-selected as people who prefer Xbox franchises; so again, more potential votes.
None of that equates to fanboyism and bias. I'm not saying it's not there; it may be, who knows. I'm just saying there are other, simple explanations at hand that don't involve fanboyism and bias, which are rather insulting accusations and just likely to stir people up.