How are Myst Uru and Myst Revalation both the fourth game in the series

Myst IV: Revelation was always the fourth game of the Myst storyline series. URU: Ages Beyond Myst was a game outside of the Myst storyline (taking place in the present time) that acted as a segue into the URU MMORPG. That URU was released after Myst III is an accident of development schedules. Myst IV: Revelation is the true fourth game in the series.