51% + 50% = ?

rounding half up

round(49.5) + round(50.5)
50 + 51

as opposed to banker's rounding / rounding half to even:

round(49.5) + round(50.5)
50 + 50

49.5+50.5 rounded?

there are different methods of rounding, and this one is likely caused by the always round up method when at a .5 value. there are methods that evenly distribute rounding, or at least attempt to.