Also seeing this behavior in OS X 10.11.5, MacBook Pro Retina 2012 (maybe the issue’s related to high DPI?), ST build 3114 with enable_tab_scrolling set to false.
Edit: It actually looks like it’s related to the length of the filename (or whatever text is used to label the tab itself). You can see this by opening a window with a bunch of tabs and opening a file named e.g. really-really-really-really-really-really-really-long-filename.txt, then dragging it. Seems like the center of the full string (if it weren’t clipped) is being used as the position of the tab.