I don't really see what's puzzling about it, you said it yourself the content would come out regardless the games success, they won't be big budget dlc's because that would have been too much of a financial gamble for a game they weren't sure would sell fantastically. The delays could have been for a number of internal reasons there's not much point in guessing that.

Also keep in mind we've had multiple additions to the game since release, the last few months have probably consisted of working on the content we got in updates as well as all three dlc's (if they weren't already focusing on other games). Chances are they're all done right now with the first 3 DLC's and they're just spreading them out to monthly release. Whether that was the right move or not is debatable but there seems like plenty of room for future DLC after these first few.