|There could be a couple of things going on.|
First, it could be as complicated as the precision with which Excel stores number internally. See here:
Second, it could as simple as the way Excel displays numbers based on the format of the cell vs. how Excel stores the number internally.
If the result of formula, or even a hardcoded number, is 6.09999999999999, but the cell is formatted to only show 1 decimal place, it will be displayed as 6.1 but actually stored as 6.09999999999999. Add 0.1 to that and you'll see 6.2 in the cell, but internally Excel sees it as 6.19999999999999.
Depending on the formula, the Formula Evaluator will either show you the exact number as Excel is storing it or just the result of the formula.
=SUM(A1:B1) will simply show the result in the Formula Evaluator (6.2) assuming a single decimal point format.
=SUM(A1+B1) will evaluate A1, then B1, (showing you the internal value of each) then the SUM (showing the exact value), then show the "displayed value".
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