I have not been able to positively confirm the issue but I have a suspicion. Your result is a number with 15 significant digits. Excel only supports 15 significant digits. There could be some numerical hiccup in there related to floating point arithmetic that is causing the last digit to vary even though the input numbers appear to be the same.
I narrowed down the issue to the calculation step of this addition
When I isolate this addition I get answers that differ in the 15th significant digit. All other intermediate results are identical. This addition adds two numbers that each have 15 significant digits.
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