When I use the formula you provided:
=SUM(10:11)
Excel calculates the sum of the values in all the cells of Row 10 and all the cells of Row 11.
Which is functionally synonymous with =SUM(A10:XFD11) ***However, this is a theoretical formula; I think Excel will automatically simplify this to =SUM(10:11)
Does this explanation fit with the output you are getting?