First, using both the SUM function and the addition operator (+) is redundant. If you want to use the addition operator, you don't need the SUM function. If you want to use the SUM function, you should use commas between the arguments instead of the addition operator.
When you use both, Excel is actually doing extra work. Excel will first evaluate the "sum" within the parenthesis by using the addition operators and then evaluate the SUM function based that single result. You can verify this by using the Formula Evaluator and watch as Excel performs the multiple steps. It takes 6 steps to evaluate your formula, but only one step to evaluate a SUM formula with commas.
With that said, there is a much easier way to construct the formula than manually entering each sheet name. For example, this will SUM C7 across all sheets from January to December, inclusive:
However, I'd still be interested to know the details behind the version issue that MervS is experiencing.
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message edited by DerbyDad03