I think it can be a bit of both.
Yes, some people are busy because they waste time. But then again, others have lots of free time because they don't do their jobs properly.
I used to have a colleague (teacher) who used to boast about his excellent time management skills, and how he never stayed too late, never took work home etc - but he really was an awful teacher, the kids hated him, said his lessons were really boring and just uninspiring. He didn't have good time management skills - he just did half a job!
I also think it will also depend on what pressures you have also got outside of work,, because that will have an impact too.
As for me, I'm always busy - partly because I do take on too much (0.6 teaching job, 2 x children with lots of hobbies, doing a part time masters degree, do part time marking, help out on PTA etc etc). But I do actually waste a fair bit of time too.