The problem with your statistical analysis is that there's a lot of stats with little analysis. That's not supposed to come off harshly - you came up with some very interesting numbers and started an intriguing topic. But those stats need some context.
For instance, take a common CB debate: Sully vs. KO. The stats you gathered paint a great picture of Olynyk and a poor one of Sully. What needs to accompany these stats is an acknowledgement of the overall lack of talent differential between our starting lineup and our bench, in comparison to the average NBA team. Sully and Kelly are marginally better or worse than one another - yet the competition they face can often be vastly superior or inferior, depending on whose perspective you choose to view it from. Sullinger faces starting frontcourts on a game to game basis, while Olynyk faces bench mobs. If there is only little separation between Sully and Olynyk in terms of vacuum production, the superior talent Sully faces will make a big difference in how one concludes who is the better player between the two based on how they do against competition of varying talent level.
TP for the topic. Just my 2 cents on how to read the stats.
OK.. I did not add my analysis since I wanted to separate the hyperbole from the facts. Still, here are a few thoughts on why the numbers are what they are:
AB - I have posted multiple times on why Avery, though a talented hard worker is a bad fit. To summarize - he takes long 2 point shots, doesn't get to the line, is a bad passer and can't man up taller SG. He is a statistical disaster.
Sully - A MONSTER rebounder, but - takes a lot of 3s and doesn't make them (could be by design which makes him look worse than he is), doesn't move the ball, is too short to guard centers and too slow to guard wings - so he is bad switching on defense.
IT - A BEAST. Was our best pick and roll player. Gets to the line a lot and makes shots. Efficient 3pt shooter. Surprisingly good rebounder for a 5"7 dude.
Jerebko - The ideal player for today's NBA. Can guard 3 positions. VERY efficient 3pt shooter. Good passer.
As for the "face lesser competition" argument that would make our starters look bad and bench look good. I agree that this should have some effect - but not as much as some would think. Yes, Sully and Bradley started more games than KO, IT and Jerebko - BUT as the games unroll, our bench and starters mixed freely (I can back this up with lineup minutes) so many times, we had 2 or 3 starters one the floor with 2 or 3 bench players vs. a similar mix of players on the opposing squad.
It comes down to fit over talent. Celtics are playing very modern game based on ball movement, spacing, 3pt shooting and quick switches on defense. The players who fit this strategy look good and the ones who don't.... don't.
Sully is not a BAD player and KO is not a great one. But KO fits our system, and Sully.. not so much. And so it goes for AB, Jerebko, Turner, Smart etc...