That magnification doesn't result in more competition,

Yes it does by virtue of having more than one choice. One choice for witness per account would result in far greater spread between witnesses, be it in between #2 and #1 spot or the 21st and the 20th because each Steem would only vote once, and each steem voting for one effectively is against all other. Don't worry if you don't understand why more than one vote creates better, more neck in neck competition, and I'm sure you'll keep thinking that it's not at all what's happening, because whales, or whatever.