For the main part the production of PL's requires that the dies are ground before being put into use. Philly used to do this with a few dies from time to time but apparently no longer do.
I know I'm splitting hairs here, but don't you mean "polished" instead of "ground"? Yes, it's a matter of degree, but... I mean there's tooth polish, and then there's that stuff they do when you get caps.
The fields of the die are irregular after being hubbed. There are waves and areas at different heights and angles. In order to show the reflective surfaces that copmprise "PL" the entire die has to be ground flat. Most dies recieve no further processing after being hubbed and are put into use. I wonder why some dies are ground at all. Perhaps there's some flaw seen after hubbing or they just want to make some coins with the distinctive look. Philly has been preparing extremely few dies this way for half a century. When they do grind a die it's usually not ground all the way flat. The 1974-P coins are virtually impossible to find with any degree of PL and when they are it's quite subtle. Back in the '40's and '50's Philly PL's don't seem so scarce.
Are all dies still made in Philly and shipped to the branch mints, as in the old days? If so, they might be polishing off minor shipping damage at the branch mints. (?)