If you convert the other way, I think it is more obvious. The pretty standard barrel lengths in Europe for both shotguns and rifles were and are 600, 650, 700 and 750 (mm). Converted to inches these are all within a tenth or so of 23.5, 25.5, 27.5 and 29.5 inches. I presume rough barrels were sold back and forth between all the suppliers and the English finished them at the odd numbers so either British or European lengths would be long enough to finish out.