Well spotted. This seems a straightforward case of two different people being conflated. There's no good reason for concluding that the Mary Green who was supposedly baptized as an adult in 1796 [without an image I even doubt this record, why would they have a precise date of birth for a 40 year old?] with the Mary Green who married Richard Marsden in 1780.
I would suggest splitting this profile into two different people and adding a note.
Edit: Hmm ... OK I've now completely reversed my position, sorry. The details on her daughter Mary Ann's baptism are compelling but I just feel that they are the same person, despite the use of her maiden name, as unlikely as it appears.
There'll probably be some strange religious reason why she was baptised under her birth name after 40 years.
Can anyone find an image of the 1796 baptism they could share?