Just something I noticed, but some pick ups have exposed pole pieces, while others do not. Is there any difference? Also, I noticed that some pickups that have exposed poles have a different number of ppieces than do others. for example, my Squier P bass has exposed poles, two poles to a string. The pups that come out of my knock-off P bass have exposed poles, but only one pole to a string. The EMG selects I dropped in the knock off P do not have exposed poles. What is the difference, if any?