For 2018 I don't know how they made decisions. There were a couple of standouts at each position (easy decisions there), but after that there is a lot of luck and randomness in the process. It it the same for every tryout, in every sport.

Examples of randomness:
-- an evaluator might be looking somewhere else when your son does something great, or speaking with another evaluator and not paying attention
-- that same evaluator might see your son make a mistake (all the boys make errors, but he didn't see or recall the good things)
-- your son is a standout for an evaluator that grades harder than the other guys at other stations (so, your son's 7 at station A looks low (even if is the highest at that station), compared to the 9s at station B, where your son might have made a mistake)
-- the evaluator that liked your son just isn't as vocal during the selections
-- a vocal evaluator has 4 kids he's pushing for, but gives up after he gets his first 3 (and your son is number 4 on his list ... even though he might be the next best player)
-- and yes, all other things being equal, the evaluator will remember the kid with yellow cleats, or a florescent green undershirt, or an "orange" team helmet (it's human nature), if that boy is performing at the same level as your son.

Any combination of the foregoing would mean your son isn't going to get the call. It is a flawed system, that isn't perfectly fair, but in the absence of something better it isn't necessary unfair.