Armatures have only 3 modes(Object, pose, edit). In object mode by default blender assumes Tab for edit mode and Ctrl+Tab for other modes. Since the only other mode for armature is pose mode blender just switches you to pose mode. In the case of a mesh there are several other option so the pie menu pops up. Now in pose mode tab is for edit and ctrl+Tab switches to the only other mode. Now in edit mode blender doesn’t know which mode you want to switch too. So, it pops up the Pie menu. There was some discussion about tab goes back to the last mode you where using and ctrl+tab going to the other mode, but I guess they decided to leave it as is.
