I don't understand it, either. I checked YouTube last night, and there were several instances of the starter turning in reverse. With most of them, the fix was to disassemble, check the brushes, etc. One mentioned to be sure that the casing was replaced in the exact location that it was removed. So I disassembled it, turned the casing (talk about a PITA), and tried it. Still turning the wrong direction. Did it again, turning the casing, and this time it spun the correct direction.
Don't know how to explain it, but it worked.