Well dev model =/= dev timeline. And the first xpac for Civ V was likely behind (internal) schedule when they shuffled Shafer out so by all accounts the original plan may have been a 1 year xpac all along.
That's a good point - Civ 5 actually looks like the exception rather than the rule in some regards: it looks like the expansions for 3 and 4 were out within about a year of release, same with Civ: BE. And X-Com 2 was a year and a half, which would be about March for Civ 6. Those look to all have consistent design teams at the helm, while for 5, if Schafer left and they had to go back to the drawing board, that could've knocked the expansion back.