It's just vibes, man. Everyone is different. Some people pride themselves on being as tight as humanly possible, for example Eminem's flow is generally completely tight with the beats, i.e the stressed words will always sit exactly on the snare drum and whatnot.
But then you've got folks like E-40 who are never on the damn beat. E-40 sort of flows randomly around and in between the drum patterns in no particular set way.
It honestly can be anything and it depends on the person and on the beat. M.I.A, as another example uses very tribal beats sometimes that are based on rhythms not generally used in contemporary music, as a result her flows are sometime so unique that they just sound nonsensical to your average person.