I am not seeing that here unless I hold the option key while filling down. Without the option key, it fills down the font along with the formula. With the option key, it fills down with the formula but uses the font of the destination cell.
An exception is with text formulas if they are in cells set to the default table style font (Helvetica Neue regular 10pt usually). In this case, a text formula will use the font(s) of the text-formatted cells referenced in the formula. If one row is referencing cells that are Times New Roman 10pt and another is referencing cells that are Times New Roman 9pt, the two formulas would look different. If you change the font in the cells to something other than the default, the formula results will be in the font you selected.
If it is neither of those things, can you provide some more detail about your table (and if the formula is a text formula) so the problem can be reproduced (hopefully).