Rolling boil has barely any steam?

I'm trying to reduce a sauce I've got but I noticed something interesting.

When I raise the heat so the sauce is at a full rolling boil, there is barely any steam coming out so I presume there is little reduction happening.

However, when I reduce the heat to a slow gentle boil, there is quite a lot of steam coming off, so there is a lot more reduction happening.

Why is this the case?