Should the taxes when using a 2-4-1 voucher be higher?

Just booked my outgoing flight to BWI from Edin and taxes are £748 for both of us in CW, our return date isn't available just yet but advised it would be much the same for the taxes.

Was on doing a dummy booking to see what the actual fares would be if we just paid for 2 CW returns in full and I noticed the taxes were £549 pp, which is quite a bit cheaper than i'm paying with the voucher.