I'm going to take a EWAG (Educated Wild-Ass Guess)here and say that it probably takes a little while for the Oxy to take effect (say an hour or so) but it works for 8-12 hours. APAP*, however, can start providing relief in as little as 30 minutes, but it only last for 4-6 hours and for moderate or higher pain can only take the edge off. So, combine the two and viola, something that's fast and long lasting.
Now, as for the difference in APAP* levels, more pain requires more APAP to take that edge off. ( Same as taking a
Tylenol (325 mg) for that mild headache and the Tylenol ES (500 mg) for that hangover from last night's party. As far as the tablet vs capsule thing, I think it's mainly a preference of the manufacturer and that there's no no real difference. ( Just like, he likes chocolate but she likes vanilla, it's still just ice cream; both are made basically the same way, and both digests just the same.)
* APAP =
Acetaminophen (Tylenol)
I might not always be right, but I'm never wrong.