Why is Anderson not a councilor?

In both my ME1 and ME2 savefile I had anderson as a councilor. I just never liked Udina.

Yet this choice seems to have been ignored when importing my save.

Is this an error, or is the choice explained somewhere?

If you appointed Anderson to the Council, he steps down later to prepare for the Reaper invasion. You can read up on this in the codex:

After the Alliance victory in the Battle of the Citadel, Anderson briefly served as the Citadel's first human councilor. He soon became embroiled in a Cerberus plot to abduct his friend Kahlee Sanders, however, and learned that he was unable to live a life without action. He stepped down as councilor and returned to the military to prepare for the Reaper invasion. The Alliance Parliament named Donnel Udina as his successor.

The events mentioned in the codex take place in the book Mass Effect:Retribution.

In the novel Mass Effect: Retribution, Anderson hands over the political negotiations he was in charge of to Udina in order to assist Kahlee Sanders from the Grissom Academy with locating Paul Grayson.

If you had chosen Anderson to be the Councilor, it's mentioned in his codex entry that he transfers his office over to Udina as part of these events. With the pending Reaper invasion, and since he prefers the military life anyway, there's no reason for him to have attempted to retake the councilorship. You therefore rightly find him in military uniform in Mass Effect 3.

He steps down. But also its interesting to note if he was counsilor then his uniform displays 3 gold bars and 1 red on his shoulders and if not he has 4 gold bars