Do you think its a coincidence that 1"PVC and 1" Brass Nipple both have an identical OD of 1.315"????

It is NOT "coincidence" it is because they are both made to pipe standards which means ALL "schedule" pipes have the same outside diameter to make them compatible with fittings. The various pipes will NOT necessarily have the same internal diameters, however. The same applies to "CTS/copper tube size" PEX, CPVC, PE, PB, etc. which means their o.d. is the same as copper, even if the i.d. fluctuates according to the wall thickness.