In my system the purchasing group is commented in the flow logic,
probably in your system too.
just execute SE51
enter SAPL0PP3 and screen 0061
Thats the reason why you can't see it on the screen.
It can either be something left from old releases like R/2 or something special for a industry solution that you haven't purchased.
If I search for OSS note using the field name T024D-EKGRP then I get a couple notes for IS-R and talking about store orders.
Reading some text makes me think that the purchasing group from customizing is taken when the purchasing group is not entered in material master.