That’s not how it works. Randoms don’t know what their mates have banned, so they may double vote for the same maps, and they may also make contradictory choices (3 guys favorite Arabia, last guy bans it). On the contrary, premades can mutually agree to ban all water maps, or all clown maps, and basically only have a subset of maps to learn, whereas a random will still have to prepare for every type of map.
I agree the percentages don’t make sense but it is obvious to me that premades get more ban options and have more chance to get their favorite map, and it is indeed unfair.