ZSE Controller described another ERAM anomaly where it depicts an aircraft in a different position than it actually is.

Narrative

Aircraft in FLAT track and route line/aircraft position symbol were coincident. Re-routed aircraft direct RBL and all was normal. Upon selecting previous route and selecting enter the route line moved 35-40 NM away from and ahead of the actual aircraft position. For some reason ERAM incorrectly thinks the aircraft should be in a different position other than the actual. Upon restoring the previous route I would expect the flight plan to be restored to the previous condition and the route line and actual aircraft position to be coincident just as it was prior; but that is clearly not the case here. The aircraft should be switching from FLAT track to FREE track as well. Recommendation; find out why ERAM places the route line so far away when restoring previous route when it should be simply undoing the previous amendment. Find out why it is not making the route line and actual aircraft position coincident with each other since it was previously that way. Need to have ERAM simply restore the exact previous condition when selecting previous route.