A numberline might be a helpful way for her to "see" this relationship, MegMeg.
I recall a lot of mystifying (well, to me, anyway) little things like this when DD was 3~7 yo. Some of it was unexpectedly more age-appropriate development overlaid with NON-age-appropriate cognitive development. It sounds like the same basic phenomenon. Predicting series was another thing that DD had a horrible time with, as I recall. It was a very peculiar-seeming "blind" spot, because she could do things all AROUND that one particular skill. KWIM?
