Firstly just to be correct, note that it's the difference in number, not the absolute number. And considering that, I'd say it might (to a small degree) come down to races for them: Most Isotropic games are 2 player, and when you're racing for Cities, or Highways, the difference will always be a multiple of two (you have x of them, your opponent has 10-x, meaning the difference in no. is 2x-10). And in those cases, the winner is often the person who got more of them.
Of course, not every game with those cards will be a race, and when it isn't, it probably means they aren't so good (and so the win rates are lower for odd numbers, which rarely occur in a race), or that while their is a race for them, it isn't so vitally important, and so there are still some left in the supply: In that case, again, odd numbers can occur. But in the key cases, when they're a vital card that sells out from the supply, you'll always get an even number difference, and the person with more probably wins, forming this pattern.
In short, this pattern is a result of strategies, not a cause of it.