The Lemming is under the blockers turning area and can climb into the middle space of the blocker where no turn area is placed.
A blocker has a turning field at the left and the right arm, but no turning field in it's middle. This allows blocker placements under hatches and falls without the lems that fall down afterwards turning. Which is really conveniant in a ton of cases.
Also there had to be a rule how low the turning area actually is. On extreme slopes like this Lems can actually slip under.
Look into the true physics mode and you'll see it live with displayed blocker turn areas!
Back in the days we decided that we mostly prefer the lighter blocker behavior --- a no-turning gap in the middle as you see here, as it's more convenient in a lot of cases.
Stepping up steep is the only case where it can be slightly annoying sometimes when you want lems to turn, but even this behavior can be/ is used to make interesting levels around it
I still post this even Nepster was a min earlier