I think part of it is small sample size (giving up 128 and 117 points in 2 of the 5 games), and part of it is scheduling. 3 of 5 on the road, 2 of 5 have been the 2nd night of a back-to-back, and only 1 of 5 has the opponent been coming off a back-to-back (and that's CHA, the one opponent the C's have held under 100, barely though at 98). 2 of the 5 have been home openers for the opponent. Also 4 of 5 have been against probable playoff teams. And has any opposing team been missing key rotation players?
As the season goes on, teams won't be so fresh and Boston plays some more bottom feeders, things should even out some.
Through 5 games for Boston, and 3-5 games in for everybody else, this is how the rank in various stats:
Opponent 3P% - 23
Opponent FG% - 5
Opponent ORB - 29
Opponent TO - 26
Steals - 22
Blocks - 4
DRtg - 29
It's still way too early to jump to any conclusions, but for a team that prides itself on defense and that features 3 perimeter oriented All-Defensive candidates (2 of which have been available at all times), to have the C's rank so low in Steals and opponent TO (ranked 2nd in both last year) and opponent 3P% (ranked 4th last year) is a little disappointing. But that's only because I always hope for the team to set the world on fire and go 82-0. I still expect them to be top 5 in those categories by the end of the year.
Too early to worry.