You've confirmed that the tab order is correctly set up
There are no script triggers such as OnObjectEnter, OnLayoutLoad or such that are moving the cursor to that field for you
Then you may have a damaged layout and you may need to either replace the layout with a copy built from scratch or you may just need to delete and replace a few of the fields--you can experiment with that and see if it works.
Better make sure it is NOT in the tab order, sure sounds like it is :)
when fields are very close to each other, I have selected the wrong arrow to indicate tabbing and had unexpected results