Originally Posted by movdqa
Head measures size from the outside. To compare with other companies, you should measure from the inside.
That's not the reason. It has to do with the metric to Imperial conversion (sq. cm. to sq. in.) and the fact that Head wanted to use nice round and symmetric numbers for it's marketing, such as 600, 630, 660, etc. even though the frames didn't actually measure 600 sq. cm., nor 630 sq. cm. nor 660 sq. cm. They are all actually smaller. But when you covert 600 sq. cm. to sq. in., you end up with 93 sq. in. even though the actual head size is 89.5 sq. in. or 578 sq. cm. Same when you convert 630 sq. cm. you end up with 98 sq. in. even though the actual head size is 95 sq. in. or 613 sq. cm.
PC600 and PT630 sounds a lot better for marketing than PC578 or PT613. Same for the 660.