Altars USED to be constructed with the altar in the west and the nave in the east. The mass would be said facing the congregation, and at the consecration all the laity would turn and face the east (towards the door in the east, from which direction Christ would return). With the advent of ad orientum in the mass, the construction of churches was altered such that the altar was built in the east and all the people faced east for the duration of the mass.
As time went on, construction of churches in a particular orientation became more difficult due to land usage and planning. As such, for hundreds of years churches have been built with “east” being understood to mean the “liturgical east”. That is, for purposes of worship, we consider the altar to be in the east, even if it is not physically so.