True. The addition operator (+) and the subtraction operator (-) have
the same precedence in XL, so they are evaluated stepwise left to right:
=C10+C21+H10-H21
is equivalent to
=(C10+C21)+H10-H21
is equivalent to
=((C10+C21)+H10)-H21
To achieve your second example, you need to use parentheses.
For more, see
The order in which Microsoft Excel performs operations in formulas
in XL Help.