I mean, if all you have to go on in a hypo is the statement "O to A . . . etc." that doesn't necessarily imply that C's interest was acquired at the same time as A's and B's regardless of when it vests, so I guess that makes sense. Now, if the hypo says that this was conveyed in one deed, I don't know what to say because I've never seen anything that says that the time at which the interests vest controls, as opposed to when they were created.
Although isn't C's remainder an absolutely vested remainder, anyway? There's no condition, and it's not subject to open.