Inherit Scale = IS
Inherit Rotation = IR
If IS checked and IR unchecked, a bone inherits scale zero to infinite. Blender takes negative numbers as positive's. So, X scale of -1 (*) is 1.
Unless, IR is checked (???). IR and IS checked, the bone inherits scale correctly.
The other weird thing is that, if IR is checked and IS is unchecked, the bone only inherits the scale of -1. No other value.
What the bone should do (what I think):
IS checked: the bone should inherit scale from negative infinite to positive infinite (obviously, including -1)
IS unchecked: it shouldn't scale at all.
IR checked or unchecked: just rotation stuff.
(*): I use it to flip planes with images, horizontally.