It seems to me that barb manufactures don't completely follow the rules. 1/4 npt fittings have a .504" tappered outside diameter. Since its the outside diameter thats critical for the fit, they use the 1/4 npt label even though the I.D. is 3/8". 3/8NPT barbs have a .675 outer threaded diameter while the I.D. may be 1/2" or whatever the manufacture chooses.
While this is a complete guess, it's the only thing that makes sense to me.
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