The 300ci I-6 was only available in trucks and never offered in the Mustang. You are probably thinking of the 250ci I-6 since that was the larger optional engine over the 200ci I-6 base engine in 1970. The 250 became the base Mustang engine in 1971.
Engine displacement is the measure of the cylinder volume swept by all of the pistons of a piston engine, excluding the combustion chambers .
It's reasonable that they would adjust the numbers to avoid marketing confusion. It's not just math, as the displacement would often serve as a "name" for many engines. I'd be more concerned about the performance not measuring up, but a lot of these engines gave more power than what was advertised.