I guess I will be first to respond.... But my first inclination is that the different POI for each ammo type has much less to do with the material of the case, and everything to do with the total combination of all components and their differences between each ammo type. You didn't provide any detail on the brass cased load.... And I certainly don't have the load data that Wolf uses in their store bought ammo... So with the information given it's impossible to compare differences in primer, powder type/charge, and bullet weight/type.
If all of the other components were identical, I'm sure you would still have some degree of difference in your POI. Of course, since steel cased ammo is not normally reloadable, such an experiment would be problematic.
Hope this helps.
Tom
ETA: on rereading your post I did see where you noted you were talking about 55 grn bullets in the brass case.