For comparison's sake, consider the great 49ers run from 1981-1998. Five SB titles, regularly winning the division and going to the playoffs.
Here are their numbers during their SB-winning seasons vs. their non-SB winning seasons:
SB-winning Seasons
- Regular season: 26.9 ppg, 376.0 ypg, 1.5 to/g
- Playoffs: 34.2 ppg, 402.8 ypg, 1.7 to/g
- Difference: +7.3 ppg, +26.8 ypg, +0.2 to/g
Non-SB-winning Seasons
- Regular season: 26.5 ppg, 377.6 ypg, 1.7 to/g
- Playoffs: 20.1 ppg, 333.2 ypg, 2.3 to/g
- Difference: -6.4 ppg, -44.4 ypg, +0.6 to/g
Compare that to the Patriots':
SB-winning Seasons
- Regular season: 24.1 ppg, 325.9 ypg, 1.6 to/g
- Playoffs: 24.2 ppg, 332.9 ypg, 0.7 to/g
- Difference: +0.1 ppg, +7.0 ypg, -0.9 to/g
Non-SB-winning Seasons
- Regular season: 30.1 ppg, 388.0 ypg, 1.2 to/g
- Playoffs: 25.1 ppg, 359.7 ypg, 1.9 to/g
- Difference: -5.0 ppg, -28.3 ypg, +0.7 to/g
Now let's compare their average regular season numbers in their non-SB-winning seasons with their playoff losses in those seasons:
SF
- Regular season: 26.5 ppg, 377.6 ypg, 1.7 to/g
- Playoff Losses: 14.9 ppg, 313.4 ypg, 2.8 to/g
- Difference: -11.6 ppg, -64.3 ypg, +1.1 to/g
NE
- Regular season: 30.1 ppg, 388.0 ypg, 1.2 to/g
- Playoff Losses: 17.0 ppg, 383.0 ypg, 1.7 to/g
- Difference: -13.1 ppg, -5.0 ypg, +0.5 to/g
So the pattern is very similar. Massive offensive underperformance in the playoffs from two of the most phenomenal offensive machines the league has ever produced. It just happens, even to the very best of them.