I don't know about this version of the software but in version 5 the default tabbing order was left to right, top to bottom and if fields, checkboxes & such were positioned slightly out of alignment, or were higher than other items, then the default tabbing could be a bit weird. Depending on how you go about setting up a custom tabbing order this could come into play. We even change the size of the background area of some objects, such as check boxes, to affect the tabbing order or how far a screen scrolls when tabbing into a check box.
Not having used this software I'm guessing but it is possible that using subforms could affect things depending on how the individual fields and the subforms themselves are aligned.