Why does portable air conditioner increase humidity?

I have an LG LP0910WNR portable air conditioner. I have it vented to a window as required. The problem is, in my bedroom (where it is located), my humidity level with go from around 55% to 80% overnight! The humidity levels definitely go down when the unit is cooling, but when the desired temperature is reached... show more I have an LG LP0910WNR portable air conditioner. I have it vented to a window as required. The problem is, in my bedroom (where it is located), my humidity level with go from around 55% to 80% overnight!

The humidity levels definitely go down when the unit is cooling, but when the desired temperature is reached and the unit switches to fan mode the humidity starts to climb. So when I wake up in the morning I get a nice 70 degree room with 80% humidity (yuck).

This is the second portable unit I have bought that I have experienced this issue with (other was an Everstar), is this the norm for portable units? Should I just suck it up and go get a window unit?
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