Records can be deceiving over small samples because of variance in quality of opponent and results in close games, among other things. Since the start of the Cleveland series they're only +59, or +4.2 per game, compared to +11.6 during the regular season. Take out the Porzingis minutes and that drops to around +34 and +2.4, respectively. Dallas, by comparison, is +2.3 since then despite playing a much tougher schedule.
Some of that is skewed by last night's game, which is probably a bit of an outlier for a number of reasons. But I also think the extra days off have helped Boston with Porzingis being out and Horford being relied upon to play more minutes. These teams aren't that different once you give Dallas a guy to help off of and a 39-year-old that actually starts to look 39 to attack on the other end.