Not 100% sure it's part of the syllabus they just assume we know we do our BOP work under floating exchange rate.
BUT I imagine it's due to the fact that in a fixed/pegged exchange rate the central monetary organisation purchases and sells domestic currency on the FOREX market independent of the goods, services or income account therefore there are currency inflows or outflows that can't be evaluated on the BOP. With a floating exchange rate, the central monetary organisation sort of pulls the strings and leads others to do the financial flows (always under a category of the BOP)