It seems that the option subtotal in subtotal module didn't preview the use of core module multicurrency. When creating an invoice in a other currency than the default, it sum only the Subtotal (excl.) [default currency of the company] but not the subtotal (excl. in currency). oer [invoice defined foreign currency by module multicurrency].
I am not a coder, but I presume to correct this bug anywhere in the code has to be defined: If module multicurrency is activated, then add additionally the subtotal (excl. in currency).