If I put these values and formulae, I get a zero total, which is what I expect

a1: 123.456
a2: 234.567
a3: 358.023

=a3-a2-a1 (gives zero exactly)

If I put this, I get a rounding error:

=(a3-a2-a1) (gives 1.42e-14)

In another spreadsheet, the error was much larger (2e-9).

Excel should give the same result either way, but it doesn't.

Does anyone know why?

Jason