When saving for web (Legacy) the object offsets itself outside of its artboard?

I dont know if I am doing something wrong, I would be grateful for any advice.

Im creating buttons for the web, the artboard frame is 202x62px, slightly larger than the yellow rectangle itself that is 200x60px. As you can see below, the yellow rectangle fits comfortably inside the artboard frame:

When wanting to save for web (legacy) this happens :

It oddly offsets itself leaving no transparent border on the bottom, here is a zoom up:

I dont understand why. 

Would you happen to know why?

Kind Regards
S.