Formatting a cell is how to show its contents. When you are writing to a cell, Calc needs to understand what you wrote. It uses the cell's formatting to see if what you entered matches the format; but if it does not (like in this case: =15.50/12 does not fit to a US number mask, which does not include any = or /), it starts interpreting the string using normal (locale-dependent) means. So - only your locale separators in formulas.