Answer:

Fairly common mistake is to use the wrong variable value from the report when performing hand calculations.

For example:

Reported Design pressure near the top of the image indicates P = 0.144 MPa.

Lower in the report, section "External Forces and Moments" variable "P" was recalculated to a different value ( P= 0.396 MPa ).

Under the "Gasket and Bolt calculations" section, calculations for both "H" and "Hp" used the updated value of "P" = 0.396 MPa

Please refer to the snapshot below for more details.