Winning is all a matter of perspective.
Yes, you are correct. Different people get different assignments and as such it's not entirely a fair playing field. At least the best that I can tell.... The highest for a single assignment that I've heard about is 3000, although I've never seen it. I had a 2500 one time this time around, but the first week I didn't see anything near that high. Most things were 1500 and under.
Now, as far as that perspective I was talking about. I've seen someone else post a picture of their standings and the highest person (the #1 spot) was only around 6500. In mine this week, the highest person got 16,900, the 5th spot was 12980. I ended in 14th at 10450 and advanced to Suburban League with a Big Prize. Did I win? I certainly think so. That's 2 keys and 10,000 simoleans I would not have had otherwise. To me, I won. I don't need that #1 spot, although, if I were in that other group, I would have had the #1 spot. Does it bother me that someone with 6500 got the top spot while I got 10450 and was 14th? Not in the least. I really don't care. A Mayor's contest, to me, is a side quest to playing the game and doesn't really matter other than what keys I can gain since they're so rare. I got 2 keys, so I won. It's all a matter of perspective.