tactical tailor fight light plate carrier sizing question

This rig appears to be sized to ESAPI plate sizes.

Trying to translate ESAPI sizes to actual measurements for civy armor (e.g. 10" x 12"). Is wiki-crappia info GTG regarding measurements for ESAPI (https://en.wikipedia.org/wiki/Small_Arms_Protective_Insert)? If yes, why are civy plates different measurements? Is there a ‘nominal’ consideration, similiar to how a 2 x 4 isn’t really 2" by 4" ?

Mods: if this is the wrong sub-forum, pls move and update with correct link.