After Gaius' defeat, someone must have made a stride towards the throne. Throughout all these patches, the new Emperor must have been consolidating his power.

On the other hand, Galvus theme was stated to be immortality on the website. Immortality isn't worth much, if you don't attain youth as well. If he is a clone or some Ascian trickery is also a likely hypothesis.

Plot wise, the first hypothesis works best.