Most clubs will not draft a player in the top ten rounds absent either a rock hard (gentleman's) agreement or a compelling reason (e.g., Mark Appel as a senior draftee). While each year a few top ten rounds aren't signed (Aiken, Bickford, etc), the penalty associated with a non-signing (the entire slotted amount is deemed used) is pretty severe. When a top ten rounder isn't signed, it's usually an indication of some kind of a breakdown in either internal club communications, advisor/club communications, or a late learned fact (usually health related).
There just aren't a lot of moving parts for the top ten; the club knows what it's max total is, knows [as best the parties can know] the players bottom line, tries to gauge the players true priorities, and acts. A miscalculation can be bad (although the compensatory pick for not signing a high draft pick does take the sting out of a potential miscalculation). The Astros lost three HS pitchers last draft because of this - two in the top ten, the other lower.
Outside the top ten, however, the calculus changes. Here there are many more moving parts which can be assessed with the luxury of time (all needs to be known by the signing date, not by the draft date).
To me, most importantly, the amount which can be paid without penalty is a number in constant fluctuation until every top ten is signed. So, for example, a HS player may state his number, and the club will not know if it can pay that number for weeks following the draft. On the other side of the coin are the HS players who for whatever reason (college no matter what, bad advice as to what he is actually worth, deemed not ready yet to be on his own, injured too close to the draft to evaluate the extent of the injury) price or otherwise remove themselves from the market and will not be drafted.
Of course, during the pre-draft scouting process, scouts collect many more names then can be drafted. It seems that many parents and players feel that being scouted = going to be drafted. For top ten rounders, cross-checkers and above will have seen the player; for the rest it's highly probable that ONLY the area scout has seen the player. For those who ONLY the area scout has seen, there is a battle royale in the draft room to get your boy picked over another area scout's boy. So, one area scout may feel that his boy will be drafted (and communicate this to the player and parents in the home visit), another area scout feels the exact same about his kid, but the club only needs one slow footed monster DH. Thus, what may appear as disingenuous to the undrafted player ("I was told I'd be drafted but wasn't"), wasn't at all personal to the club ("how many slow footed dh's do we really need").
In the 2010 draft, my S was called by half dozen clubs and told he was next up on their list. He's still waiting for ine of those clubs to tell him to report.