Manifold pressure not reducing properly when throttle pulled back

You have a display issue!
With the engine OFF, your MAP will be the altimeter setting minus the field elevation factor of  ~1" per 1000"
Example:  Assume the altimeter setting is 30.00" and the field is at 2500 MSL.
You should be seeing 30" - 2.5" = 27.5" MAP
Unless you are in the mountains during a Hurricane Your MAP on the ground will always be well above 25", NOT 10" !
In Seattle WA, it will always be >29".

15" at idle is a reasonable number though.
The pressure should DROP significantly after starting and idling the engine, it will NEVER go Up!