That's center speed. The higher the number, the less overall recoil. It's a measure of how fast it can return to center after each shot.
And also, when it's 40 left 40 right, it doesn't mean they cancel out entirely - each shot individually kicks in one direction or the other, so on average they should generally result in a spread around a center point, but it could be randomly offset by a bit just due to probability.
When it's 10 left 60 right you're practically guaranteed to kick overall to the right, but individual shots can kick slightly left.