Skeleton horses don't get their movement speed, health, or jump strength attribute randomized on spawn. This is inconsistent with other horses.
This is likely due to the entity id change of skeleton horses in snapshot 16w32a as it wasn't present beforehand; someone probably forgot to extend the random spawn bonus to the new entities.
Affects 1.16 pre3
I've tested this on 1.9 (version where skeleton traps were added) and my result was that Skeleton Horses (EntityHorse back in 1.9) were having randomized MovementSpeed values.
Also, I've tested out in witch version was this problem "created" and I have noticed that in 1.11 it already happens, but in 1.10 no, then, it is very likely that the ID change of the skeleton horse of the 16w32a is the cause of this bug.