This is because the spelling reflects the origin of the word not the pronunciation. There is no "one" correct pronunciation. In fact you can almost predict that whereas the Asante Twi pronounces "gu" as "dw" the Fanti and Akuapem pronounce the "gu" as "gu".

So if you want to read the following words in Asante Twi you'd always use the "dw" pronunciation.

oguan (abowa a osu mbɛɛ mbɛɛ)
eguaree (bea wo guare wɔ)
akongua/egua (ade a nimpa tena so)

etc.