I recently bought a 627PC with the 2 5/8" barrel, and have wondered why it was not initially or subsequently made in 3". Round numbers just make sense, in a way. Suspect the 2 5/8" is the minimum length that allows the ejector rod to kick Magnum cases clear of the cylinder.
But, while 3" in a K frame or especially a J frame may result in a nice balance, the extra 3/8" inch really means nothing on this gun, as the barrel and shroud are contoured and "see through" and weigh less than other designs. And the N frame has too much weight in it for the balance to change by adding a smidgin more barrel.