Talk about ignorance on the side of the city.
I don't think its ignorance as much as it is a way to make it fair for everyone to bid. You guys are looking at it from the contractor perspective. I've got hundreds of thousand of trees to manage, I don't think you can appreciate the kind of paperwork that would be generated from getting bids on individual trees!
I base mine on 10in increments (0-9, 10-19, 20-29,etc) Then depending on my projected budget I can tweak the unit #'s. I also make it clear that the unit numbers that are bid are for bid purposes only, meaning if you put 40'' removals at $500. because there is only one listed on the bid sheet, I can give you as many 40'' as I want.........
I had a guy come in and wipe out $350,000. worth of work in about two months, not a bad little score imho. He based his prices on the theory that there aren't too many trees over 40'' in the city (he was VERY wrong) so I got a good low price for trees like that but at the same time he was getting $200. for 3'' trees. His secret was keeping the mid range trees high. I am definately on the wrong side of this biz when it comes to making $$$$$$$
Hope this makes sense. And please don't hack on the Cities. There are all kinds of rules and laws about buying goods and services that you probably just don't know about.