Why? Because shooting low means you aren't tuned. Move your rest up just a hair and try it (or move the nock pt down a hair). It takes less effort to move a nock pt than adjust a sight, IMO. Think of it like this...your bh tipped arrow wants to travel in the exact direction it is pointed when you release. So, if yours are going low, your arrow is starting out pointed down. Move the rest up, or the nock pt down to correct.

A field pt tipped arrow is much, much more influenced (corrected) by your fletchings, so they often will not reflect the exact direction they are pointed at release. Also, they will most certainly group ok even out of a poorly tuned bow, simply because the fletching can keep them under control and allow them all to go to the same location on the target.