Over the years I've boiled a fair amount of wood, and noticed the water level drop, drastically after I turned off the heat.
I didn't know what the cause is, but some people have claimed it was the wood absorbing water, other said it was water still coming off the pot due to the heat.
I did a little test this time, I rough turned some wet/green Madrone/Arbutus I left some of the rough turned wood in water, for days before I boiled. I also left some just sitting on a pallet, next to the un-turned wood.
Just before I tossed the wood in the boiler, I weighed both lots.
After boiling the wood, I let the pot cool down just like normal, then took the wood out and weighed both lots.
On average the wood that had been left in water before boiling did not change weight.
All of the wood left out in the air gained weight, on average the wood gained 5%.
Just to be clear how wet the wood (Madrone) was. Before turning the wood had been simply left outside (in the rain/fog/sun/etc). But I was constantly wiping sap and water off my mask or safety-glasses, there was a constant stream of water running down the tool, and the lathe ways were constantly wet.