That's not just the best asenwr. It's the bestest answer!
There are restrictions on what units can be uncroaked. Its likely that just as nothing which can talk is capable of being harvested for rations, that nothing which can't talk can be uncroaked (i.e. if you can harvest it for rations, you can't uncroak it).
The first Uncroaked to appear in the comic were three skeletons. Ever since that point, all uncroaked have taken the appearance of the units they had been previously. There are three explanations for this phenomenon:
1) Unlike the 'standard' zombielike uncroaked seen throughout the comic, these skeleton-croaked can be made whole-cloth by a Croakamancer, with no bodies required. This advantage would be offset by their having no special abilities (like gobwins or marbits' tunnel prowess, for instance), lower stats, or higher cost in Juice.
2) These are in fact regular uncroaked that have merely deteriorated such that their flesh is gone and they are no longer identifiable. Their stats would have decreased over time to a low level.
3) Balder had not planned ahead in this case, or changed his mind at some point, and the skeletons will be Retconjured out or explained away in the future.