My PSI keeps increasing after I let water out.

I have a Weil Mclain Gold CGT.

It is a gas fired system.

The water pressure is at around 40 psi and the chart on the side of the boiler says MAWP Water 30psi. I assume that means that max the water temperature can be is 30 psi. Every time I let water out of the system and the pressure goes down to 30psi it goes right back up to 40 psi. Any ideas what can be causing this?

Thanks!