Why are some concrete posts of elevated rail systems not aligned at the center of the girder which will hold the 2 rails of the train?

Trains have very specific requirements for how much they can rise/fall or turn. As such, the available locations to place a support can easily not line up with the track. So they place the support as near as possible and provide a secondary member to support the rails.

You often see this with freeway interchanges or intersecting bridges.