[ixth]

Внимательно читаем доку:

The .offset() method allows us to retrieve the current position of an element relative to the document. Contrast this with .position(), which retrieves the current position relative to the offset parent. When positioning a new element on top of an existing one for global manipulation (in particular, for implementing drag-and-drop), .offset() is more useful.

offsetLeft/Top берется от offset parent.