I wonder if Belichick is leaning away from adding players later in the year in his philosophy, in comparison to earlier in his coaching career. For example at WR you can compare the acquisitions of players like Doug Gabriel or JJ Stokes in an attempt to bolster the position, while last year nobody else was brought in after first Lewis, and then Galloway were cut. Walter was added to the roster fairly late as a backup QB and he was cut before week one. The Pats traded for Burgess due to a desperate need at the position, and he didn't have much of an impact until very late in the season.
Just a guess, but maybe BB is now thinking that if a new player is not brought on early in the offseason, his chances of a positive contribution in that same season drop dramatically.