I really wonder if all those positive previews and win projections for the team in the offseason are the reason. The players are no longer playing with house money. Expectations are for them to be a playoff team.
I believe this is at least part of what's going on -- on both sides of the ball.
The Celts aren't taking teams by surprise anymore. Nobody is coming into games against the Celtics thinking of them as a team that had a bottom 5 record the year before and that traded its "best" players early on in the season.
Instead, this is the team that made a surprise run to the playoffs, led by a wunderkind coach, a team that's supposed to win way more games than you'd expect considering they don't have anybody worthy of All-Star consideration on the roster.
Teams have also had a lot more time to watch tape of this group of Celtics and prepare for the way the Celts like to play. Last year, opponents that were used to defending a Rondo-led team that heavily featured Jeff Green had to deal with a team that had Isaiah Thomas and Evan Turner taking turns leading a much faster paced attack.
Then you have the Celtics side of things. So many guys on this team -- Turner, Lee, Sullinger, Zeller, Amir, Jerebko -- are playing for their next contract. They are basically auditioning every night for a starting role somewhere, whether that's here or elsewhere.
On top of that we've got so many guys who have "earned" a spot in the rotation that the minutes are constantly changing. The roles for most of the players, with the exception perhaps of Isaiah Thomas, Marcus Smart, Jae Crowder, and Avery Bradley, are in a permanent state of flux.
Take all of that into consideration, and it's not so surprising that the team has struggled so far. Yes, as you point out, the record is not especially surprising given the teams they've faced. They beat one team they should have beaten, they lost to a couple teams that should have beaten them, and they lost one close game that should have been a toss-up.
At the same time, they've also looked bad. Even against the Sixers, the Celts really haven't been able to put together even a full quarter of nice looking basketball. That's concerning for a team that was "supposed" to be this year's Atlanta Hawks.