Macintosh cursors were configurable right back to the beginning; the hot spot could be anywhere in the cursor's 16x16 rectangle. The text insertion point cursor had the hot spot centered and on the baseline, the crosshairs had it in the center, and so on. I have heard that it was the same in Windows, from the beginning. So the location of the arrow cursor's hotspot in the top left is a result of that being the arrow's tip and not a cause of the arrow tip being located there.