Nothing wrong with the... nothing wrong with the maths?
Let's spell out exactly what your maths amounted to:
( sea level pressure - Unadjusted data) - (sea level pressure -(unadjusted data + adjustment))
= (unadjusted data - unadjusted data) - (sea level pressure - sea level pressure) + adjustment
= 0 - 0 + adjustment.
You just proved that adjusted data is adjusted. Well done.
