Physics to the rescue:
Contrary to popular belief you cannot see steam.
What you can see is tiny droplets of water that were steam (= gaseous form of water) before, but have condensed (= returned to liquid state) again on very small particles like dust motes. If the amount of droplets is big enough in a certain volume of air, they become visible. In other words, you have created a small cloud in your kitchen.
Now back to your pot:
- If the pot is simmering, a lot of water will condense over or near the pot, especially if your kitchen is rather cool.
- At a full boil, the steam disperses more, so when the condensation happens, the tiny droplets are spread out wider, making the "cloud" harder to see. Also, the air around the pot is likely to be a bit warmer, so that the gaseous H2O will likely condense at a greater distance from the source anyway.
This may lead to the assumption that a boiling pot emits less steam when actually the opposite is the case.
If you want to verify this, look for condensation away from the heat source, e.g. your cool kitchen windows. There, the different amounts should be very obvious.