I'm new to the forum, so I will apologize in advance if you've dealt with this question a million times already.

The defensive player has obtained legal guarding position and is standing straight up with arms straight up, preparing for the offensive player who is driving in for a layup. At the last second, the offensive player changes direction slightly and hits only one shoulder/side of the defensive player instead of making contact squarely on the chest. It seems that this is always called a block instead of pc, even though the defensive player had LGP and the offensive player initiated the contact. Shouldn't this actually be a PC foul?

Where the contact takes place should not be the issue. If the defender stayed in LGP and contact occurred in the way you described, it should always be a PC foul. Now what should be and what is called is another story.

Peace

A lot of times, when this happens, the defensive player tilts towards the shooter, causing contact. This is a blocking foul. Other times, when the defensive player maintains LGP, the contact is ignored because the defensive player wasn't disadvantaged by the contact. And yes, sometimes we get the call wrong. ;)