As one who doesn't put all my faith in a single column of goaltending statistics, what it indicates to me is that the Devils of 1998-2002 were a different team than the notorious "trapping" teams of the Lemaire years. The Devils of Robbie Ftorek and Larry Robinson played more offensively, took more chances, gave up more chances, and relied on their world-class goalie to limit the damage....But for those three seasons in they were a puck-possession, outscoring type of team. Such teams may or may not allow bunches of shots, but in my observation they do tend to give up a better quality of scoring opportunity when their players are tending towards the other end of the rink. One predictable result of this altered team philosophy is a lower Sv%, and Brodeur's were indeed merely "average" throughout those years.
Sounds plausible. However, all evidence indicates that it is nevertheless wrong. Brodeur's numbers did not appear to suffer from a team that took more chances. Here are his even-strength save percentage numbers from 1998-99 to 2003-04 (along with the league average):
1998-99: .915 (.916)
1999-00: .912 (.912)
2000-01: .919 (.914)
2001-02: .917 (.916)
2002-03: .921 (.918)
2003-04: .924 (.922)
That looks like the same guy doing exactly the same thing. Brodeur's performance relative to the rest of the league was very similar throughout the entire period. The only thing changing is that the league average is gradually rising. If his team was taking a lot more chances between 1998-99 and 2001-02, you would expect Brodeur to do worse relative to the league in that period, and better in 2002-03 and 2003-04 when the Devils were by all accounts a defensive powerhouse.
The main reason for the drop in Brodeur's save percentage under Ftorek and Robinson appears to be his poor play on the penalty kill, a situation that hardly seems to be affected by a team's offensive philosophy. Here are Brodeur's PK numbers (again compared to league average):1998-99: .864 (.872)
1999-00: .908 (.866)
2000-01: .839 (.862)
2001-02: .849 (.872)
2002-03: .866 (.869)
2003-04: .878 (.867)
Note the particularly poor results in 2000-01 and 2001-02. It seems pretty unfair to blame Brodeur's team for it either - New Jersey's penalty killers were Stevens, Niedermayer, Madden and Pandolfo. In 2002-03, even with a below-average penalty kill save percentage from Brodeur, New Jersey led the league in penalty kill efficiency.
The Devils scored a lot of goals from 1998-99 to 2000-01 (their offence ranked 2nd, 2nd, and 1st in the league). But they still were excellent at preventing shots (allowing the 4th, 4th, and 2nd fewest shots in the league). It might have been a little bit more challenging to play goal for New Jersey in this period because of an increased offensive focus, but it seems unlikely that there was a large difference. Brodeur's even-strength results are pretty consistent through the period, and his save percentage variances can be explained by his performance on the penalty kill.