What does a negative expression distance mean?

[ThomasLLi]

I am consistently getting a few clusters with a negative "normalized expression distance" value.

My understanding is that this is supposed to be a magnitude, so how can it be negative? Looking at the preprint, it appears to be a possibility if the MED of the control is larger than the distance between conditions, but I'm not sure how to interpret a negative magnitude in the context of gene expression.