I received the suggestion lately that one of the reasons for the large save percentage discrepancy between Brodeur and Hasek was that there was stronger opposition in the Atlantic Division than in the Northeast. I decided to run the numbers for both goalies to check this out. I decided to look at games from 2000-01 and earlier for both goalies, as that was Hasek's last season in Buffalo.
Brodeur: .911 vs. division, .911 vs. rest of East, .916 vs. West
Hasek: .934 vs. division, .925 vs. rest of East, .925 vs. West
There wasn't much effect for Brodeur, but Hasek's result was surprising to me. The schedule was more balanced in the 1990s, so the divisional effects we see with the current NHL schedule shouldn't have been as much of a factor.
Yahoo Sports gives a breakdown of every goalies' record against all other teams (here's the link to Hasek's page). If we isolate the rest of the teams in Hasek's division, they don't look too out of the ordinary for him, with one clear exception:
In his career, Dominik Hasek has gone 24-9-5 with a 1.49 GAA and a .950 save percentage against the Ottawa Senators.
The type of Ottawa teams Hasek would have faced are very different, from the lousy expansion team of 1992-93 to the President's Trophy winners of 2002-03. Thanks to Hockeygoalies.org, shot and save results are available for every game against Ottawa. The Senators were a low-scoring team until 1998-99, when they won their division with 103 points. It makes the most sense to divide Ottawa games into two periods: the expansion team period from 1992-93 to 1997-98, and the dominant team period from 1998-99 to 2000-01.
From 1992-93 to 1997-98, Dominik Hasek was 17-4-1 against Ottawa, with a miniscule 1.33 GAA and a .955 save percentage. That is about what one would expect for an expansion team up against the greatest goalie of all time, and it supports the theory that Hasek fattened up on some weak teams.
Except that Hasek's success against Ottawa continued even when the Senators got good. Here are his results from 1998-99 to 2000-01: 4-3-4, 1.41, .957. His save percentage actually improved as the Senators did, although his team obviously provided almost no goal support. Hasek also dominated Ottawa in the playoffs, going 5-1, 1.55, .952 in the one-and-a-half series he played against the Sens.
Playing against the Senators was not much of an advantage for Hasek, because of the balanced schedule. Martin Brodeur played against Ottawa almost as many times in this period (28 games) as Hasek did (32). From 1993-94 to 1997-98, Brodeur was 12-3-2 against Ottawa with a 1.46 GAA and .937 save percentage. The difference is that Brodeur wasn't able to handle an improved Ottawa team as well as Hasek was. The Devils lost to the Senators in the 1998 playoffs, although Brodeur played pretty well (.927, 1.95), and Brodeur then went 4-5-2, 2.96, .893 against the Senators over the next three seasons.
I don't think playing in different divisions had much of an effect on the statistics of Brodeur and Hasek. The schedule back then was balanced so their strength of opposition was relatively similar. Both racked up wins and shutouts against the expansion Senators, the difference was that Hasek continued his domination even as Ottawa rose to be one of the best teams in the league.
Sometimes players have unusual success against a particular franchise, and that seems to be true in the case of Hasek vs. Ottawa. Hasek's repeated success against Ottawa is the main reason why he had such strong intra-division numbers in the 1990s. Little wonder then that the Senators went out and acquired Hasek in 2004. They had definitely "seen him good".