Accumulation is pressure above the maximum allowable working pressure of the vessel. Overpressure is pressure above the setting pressure of the relief valve. When we say ‘accumulation’, its mean we are talking about the vessel, and when we say ‘overpressure’, we are talking about the pressure relief valve
For better understanding, see this picture
The pressure level of the vessel is presented in the following picture.
See above picture,, for non fire case, the increasing pressure of the vessel with MAWP 100 psig is allowed up to 110 psig (the allowable pressure is 116 psig if multiple overpressure protection devises are installed). That is the cause; the PSV shall be set at 100 psig and shall have relieving pressure 110 psig (or 116 psig for multiple PSV)to protect the vessel.
See the picture below
Thank my friend,hopefully this posting give better understanding why the PSV is set at 100 %MAWP and the max relieving at 110%,116% or 121%. That all because the PSV is used to protect the vessel.