His father was George Simpson, his mother Anne.
It may be that in this part of Wales in 1806 he was christened using the Welsh patronymic system,
that is William ap. (Son of) George, and free reg is trying to explain this.
I have seen suggestions that in some parts of rural Wales incumbents used patronymics until quite late.
Familysearch is no use, they have it as patronymic, but list all Welsh baptisms as patronymic until 1810.
Sorry I cannot explain it better

Mike
Edited to remove the sentence re the daughter, there are other children born to George and Anne in the same chapel, it would be worth looking on free reg for these