Because climbers stop when they hit their head on a ceiling, but splat height is measured by how far the feet fall. In other words, the fall distance is the same but the smallest fatal distance from floor to ceiling is slightly more than the splat height.
So, the Climber splat ruler is (fall distance + distance between the top of the fall and the ceiling)?
That makes sense, since we don't know where the Climber is actually falling
from; I now understand better the need for the extra measurement. Also, I can see how this situation might come about unintentionally, rather than as a result of bad design.
I do still agree with mobius though; as useful as the ruler is for this purpose, a level designed in such a way that necessitates its use is poor design. Discontinuing the ruler at this point would be too drastic a solution to this, though. Better to provide the ruler (albeit separately from the main one, perhaps), but discourage levels which can't be solved without using it excessively.
I plan to add the Climber ruler as a separate entity, callable via shift modifier of the regular ruler hotkey and/or its own hotkey. This seems to be the best way to avoid any possible confusion.