that's sort of a glitch. in that the spell "happens" faster than it takes to actually animate. so you can swing during a lengthy animation because, as far as the game is concerned, you're done casting.
you can also see this with multihits, all the hits HAPPEN at once, but it takes time to animate them all. so for example, duel wielding in aby with apoc atma.. you get a lucky 6hit proc as you hit a spell macro. all 6 of those hits landed at once, right before you started casting.. but the game may still animate those 6 swings in sequence anyway, wich can make it look like you were swinging away mid-spell. (another fun example, the first time i fought charybdis... he kept swinging after i was dead. for like a solid second)
but while the spell is charging, the delay ticker stops.
i mean i WISH it kept ticking away while we were casting, i've actually suggested that as a job trait for rdm.. but that's not how it is now.