If I remember from last year they send way too many through this round. Then the judges got together and decided to weed out a bunch before starting the Vegas rounds.
There are probably a lot that get voted through this round that cannot go to Vegas for various reasons.
I have noticed Piers will not criticize children harshly. A lot of acts are voted through because the judges do not want to look like ogres. But, in reality they would never work in Vegas.
When they finally get to Vegas it is a lot more talented group since they drop a lot of the ones that could not realistically win. Also the real behind the scenes consultants start working on the acts. You notice they get custom lighting, extras, sets, props, etc. The acts are all given a professional do over to maximise the talent's potential.