API Regular Connection in Drill Pipe/Collars/Bits

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API Regular Connection in Drill Pipe/Collars/Bits

15 January 2021 As described in the image,

"The number before REG refers to the drill pipe size on which the connection was cut".

Can anyone provide any further clarification on what this exactly means? Let me express my question with an example:

A 12 ¼ drill bit with 6 ⅝ REG PIN connection as written on it. When I measure the connection's max. OD, it's 152 millimetres (5.98 inches). But if we convert 6 ⅝ inches to mm, it equals about 168.3. The same applies to drill pipes and collars.

Why is there such a difference between the two?