There are a ton of factors- the bottom line is that the dryness, by itself, is not indicative of anything "wrong".
Birth control can affect natural lubrication- either up or down- significantly. Some women just naturally don't get all that "wet". Some get extremely "wet".
And even in those categories, there are those who seem to be always wet, those who get really wet during intercourse or direct stimulation, and those like a girlfriend I once had that would get VERY wet at the slightest stimulus, genital or otherwise- she'd get very wet if I said certain words, or from the most passive of physical contact on a lot of her body.
Also: a female's ability, or lack of, to ejaculate and her natural lubrication have nothing to do with one another.
So what you've got is a girl who just doesn't naturally produce a lot of lubrication, but does ejaculate. *shrug* It happens, man- keep up the lube and the good work. lol