Because Liverpool and Sheffield United are two of the best teams of this season so far. The Reds have only conceded 14 goals, whilst the Blades have conceded 19.
Alisson has only played 11 games, due to injuries and sanctions, compared to Henderson's 19, but their numbers are equally astonishing. The Brazilian has conceded just five goals, while his opponents have conceded 16.
Their averages are more than remarkable. Alisson has conceded 0.5 goals per game and Henderson 0.8. To compare this, Alisson finished last season letting in 22 goals in 38 games, which would make him, the keeper who conceded the least goals in the Premier League, with an incredible average of 0.6 goals conceded per game.
The Blades, despite their latest setbacks, are not far behind in terms of goals conceded. In fact, they are, together with Leicester, the second team to have conceded the least goals this season. Their numbers are better than those of the big six and are less than half of the most conceded, Norwich City.
Being one of the teams to concede the least goals is always thanks to a good defence, but also thanks to a goalkeeper who is a guaranteed safety net. Alisson, at 27-years-old and Henderson, at only 22 and called to be De Gea's replacement at United, the club where he comes from, are guaranteed safety nets for their respective clubs.