The 0 as an automiss is a non factor because the 250,000 rolls all represent hit rolls - not attack rolls. So it doesnt matter if you hit 2-20 or just 19-20, you still hit 250,000 times.
.55 is the odds you get of hitting when a 10 is a hit (11/20 = .55).
This is an odd number because it skews. If you're hitting on a 19 and upgrade to a +3 weapon so you're hitting on an 18 your damage increases by (.15/.10-1 = 33%). On the other hand if you're hitting on a 3, then the extra hit makes it a 2, giving (.95/.9-1 = 5.6%) This is ignoring critical hit damage, which skews it down, but it still explains why +hit is undervalued frequently. The tougher the monster is (and thus the higher its defenses), the more +hit adds to your damage.
That's ignoring the overkill factor, by the way. The overkill factor is simple: the average DPR for a 1st level character is something like 10. You could get the same DPR by having a 1 in 20 chance of doing 200 damage. Why would no one pick this as a 1st level character? Nothing has 200 hp. You'd overkill so badly it isn't even funny.
Smaller bursts of damage in more frequent increments are less wasteful, because less useful damage is lost to overkill. When the monster has 8 hp left, you just need to hit it - and +hit does that.
