I am not very sure, but the I think it is because the stability study for the neutral point considers only the forces and moments that appear instantaneously after a change in angle of attack. The subsequent trend determines it stability or unstability.
But when an airplane manoeuvers (pitches up, for instance) the angular rotation about the lateral axis makes the tail have a vertical downwards velocity. This velocity plus the forward TAS induces an extra angle of attack increment in the tail, which is always stabilizing. Therefore an airplane with its CG at the neutral point still has some stability when manoeuvring.
this damping is less effective as TAS increases, so for the same IAS it is reduced with incresing altitude.
Or something like that...
The experts are around here, they will explain much better when they spot this post