distance on the equator by map project

Namba Takaya <[email protected]>
Newsgroups gmane.comp.gis.gmt.user
Message-ID <[email protected]>
Dear Sir
I tried to calculate the distance on the equator to check the result of mapproject as
 lon=2 lat=0 lonn=3 latn=0 echo $lon $lat |mapproject -f0x,1y -G$lonn/$latn/n 
and the result is2       0       60.0405398135.
My question is why it is not 60.0000 nautical miles exactly?
regards,
takaya
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