Worlds of Ultima takes place in the 200 year gap between Ultima VI and Ultima VII
The Avatar does not leave Britannia for Earth in Ultima VI, but stays in Britannia and ends up on a series of Worlds of Ultima adventures before returning to Britannia or Earth. Nystul or the Time Lord may have arranged this by offering the Avatar, the Avatar's companions, and some Britannian citizens to cross through a portal, though they ended up being translated as inhabitants of those worlds until the Avatar returned to Britannia or they were brought back.
Eodon and Mars are alternate histories
Like the Time of Legends in Ultima II, Eodon and Mars are not the same history as the Avatar's history of Earth, they are alternate histories created by messing around with time and space.