TOPIC: Why does Negative SV = Overestimating time?

I have a question regarding schedule variance that I can't figure out. If I have a NEGATIVE SV, why does that mean that project management OVER estimated the amount of time needed?

I would think that if the project is ahead of schedule that the opposite is true (management UNDER estimated time needed)...
the higher the negative variance = the more ahead you are of schedule = the more underestimation.

What am I missing? Thank you in advance!