I believe that the slanted ones are thought to be in motion, and therefore more "active."

The swastika was not just found in the East. The Maya Indians of Mexico also used this symbol, as well as a cross. Other ancient civilizations of the western hemisphere, such as the Inca and Aztecs also had it, and it is one of the things which people like the Mormons like to drag out to "prove" that this continent was settled by people from the Biblical lands or China. I remember reading in a very old textbook that there are two universal symbols, found in every ancient culture no matter where it arose, and that was the swastika and the "hocker", a symbol resembling a frog or a man with arms and legs splayed.