The differences in sound between rough sand cast magnets and "polished" magnets is almost certainly due to the magnets coming from different foundries or being made in different batches at foundries with loose tolerances or sloppy practices.
You can hear the same types of differences if you get several of the "same" magnets (all rough or all polished) from several foundries, especially the inexpensive ones from offshore foundries. Many of those operate in a very manual way and the results have a great deal of variance from batch to batch and from one foundry to another.
I always use rough cast AlNiCo bar magnets but only because that's correct to what Gibson did in the 50's and 60's. Not because they are arguably better. In fact, they are arguably inferior as far as physical fit.