Our fair does this too, and it is really interesting to see the difference that management styles make. Some of the birds are half the size of others.
It is important to find the 3 most similar birds for each pen. This is a production class, so the judge is looking for uniformity. Three pullets can beat three larger roosters if the pullets are more uniform.
Weight can vary by whether they were hungry or pooped before weigh-in, which is frustrating if they were perfectly even at home but now one is off! So frustrating!
Our judge compensates by feeling the birds up a lot to judge firmness of the meat as well as uniformity of weight. These are supposed to be meat birds not fat birds, so he doesn't give points for birds that are obviously sugared up and flabby. Your judge may have a different opinion.
Make sure the birds are as clean and feathered as you can get them. The judge will give points for the healthy, prepared, finished look of your birds. We use white dog shampoo and wash them a week before and a day before.
We raise ours on pasture in a hoop house. One of these with a tarp over it.
http://www.oardc.ohio-state.edu/4hpoultry/t02_pageview/Hoop_House.htm
It needs to be moved frequently. Broiler poop is different from regular chicken poop, more of it and not as fully digested, mats up terrible on the grass. I sometimes have to break the mat up with the garden hose sprayer in dry weather.
We take feed away at night and feed only during the daylight hours. We have found this makes them eat big in the morning and also in the evening in preparation for sleep. They are not lit up at night, animals need sleep to digest food and grow.
We use a 20%-25% protein feed that includes animal protein. The Meat birds get any high protein table scraps.
The variety our fair gets from the local industrial chicken grower (nice of them to donate) has not had significant leg or sudden death problems. We are very lucky there. We have had no loses and found it easy to make 2 uniform pens of 3 each from 12 birds, but I think 20 would better if you can handle that many.
We have won champion or reserve champion broiler pen in the last two years. We were beaten by a gal who asked about and copied our method (4-H is about sharing and growing together).
Good Luck with your project!
We're getting ready to wash chickens for the fair tomorrow....we were joking about using our white dog shampoo. We can use that and it won't hurt the chickens?