The short answer is, there is a lot of fuzzy accounting that happens in the MLB Draft. Teams have bonus pools which they can allocate, and MLB places penalties on teams on a sliding scale based on how far over a team goes. On the flipside of the equation, HS players now have a fair amount of leverage thanks to NIL, so there's not a lot of risk for guys to ask for significant signing bonuses.
Make no mistake, the overwhelming majority of HS guys rated as first round picks will get drafted in the first round and will sign. However, where things get squishy is when you start seeing guys making first round demands who maybe aren't rated that high. So, what a team might end up doing is draft a bunch of guys who'll sign underslot, and then take one of those guys with, say, a 6th round pick and offer him a substantial bonus. If he signs, awesome. If he doesn't, it's not as big of a loss compared with drafting him with a 2nd rounder and losing that pick.
There's a good crop of high ceiling HS pitching still available, including Chicago area pitchers LHP Jack Bauer (Lincoln Way East) and LHP Cameron Appenzeller (Glenwood HS).