It is hard to speak for a team, know what their strategy is going into a draft.
In my opinion the Pirates didn't like certain players in the spot they were picking as much as other teams/scouts may have projected there. For one reason or another they decided that they could save money by drafting another player and paying him below his slotted value. Saving this money may allow them to save money for later rounds, as well as sign undrafted free agents.
Here is a story about the Minnesota Twins landing a undrafted player with a lot of potential.
No matter where you are picked in the draft, thereit is a slim chance of makingtough to make it to the MLB club. Story