I can see where that might happen at the tournaments but as my son never did those I can't speak to that, but my experience with the PG showcases is that they gun virtually every pitch. There's no way to tell how much a player who hasn't had new results posted for quite a while has increased their velocity in the meantime. Even when the time difference is just a month or so I've seen big jumps just because some days a kid has it and some days they don't. If you look at the kids who do the Sunshine events and then go on to the PG National the next week you'll see some who stay the same and some who make some pretty decent jumps in just one week. No way to explain that other than just being more pumped up at the bigger event.
Mine had a real good showcase one year where he worked 83-84 and touched 86, throwing only two or 3 innings. The next year he started the game and went 4 or 5 innings working 83-85 and according to the blog touched 87. He was listed at 86 giving the impression that he hadn't made any progress for a year. I didn't get the impression from speaking to the people doing the gunning that he had hit 87 or even 86 so maybe that was a typo in the blog. Who knows? Yet a few weeks earlier he had worked 86-87 at a camp throwing to only a few batters. Pitchers just plain vary and sometimes the luck of the draw as to when they are on or off makes a big difference.
Eventually, they'll have to perform for whatever team they are on and what they can do consistently is what counts.