Exceptions to the Diameter Rules
You may have had the following frustration:
You have encounted some kind of "in-between" hose fitting, where you just can't seem to fit this hose onto this hose barb. You end up grabbing a propane torch and heating the hose enough to get it to stretch onto this fitting, and you know that using the next-size-down hose barb would result in a loose-fitting leak monger of a connection... so you curse it on, knowing that it just ain't right, but hopefully it will work...
I think that what you have come across is a case of mistaken identity. This flexible thing you have is not a hose at all. It is "flexible pipe," though neither the manufacturer nor the merchant you bought it from saw fit to inform you of such. There is out there a flexible PVC pipe, often glibly called hose or tubing, which is made to fit up to the standard "socket-weld" PVC pipe fittings. This flexible pipe is manufactured to the standard O.D. of its trade size, with no special regard to its I.D., so it fails to fit well to any standard hose barb fitting.
Though I haven't confirmed this, I suspect that there exist some hose barb fittings that are made to fit to the ID of these flexible pipes, and that they may be the source of some of the frustrations I have encountered trying to mate them to standard hose. I have yet to confirm this, and I will keep the community posted on this. Please leave a comment if you can substantiate this allegation.