Well, you bring up a good point, and it serves to reinforce my recommendation to ask the questions. For instance, here is a specification for schedule 40 and 80 PVC pipe: http://www.harvel.com/PVCpipe4080spec.html
If you study it, you will see that 40 and 80 were designed to be mixable in a water system. Actually, neither one of them follows the ID specifcation all that closely. You certainly couldn't say what the ID and OD of 4 inch pipe is unless you look at the spec.
Nevertheless, traditionally pipes have been specified as IDs. You can see that in steel and copper pipes, for instance. Tubing is OD specified, mainly so that compression fittings, which fit the outside of the tubing, will fit.
Flexible hoses don't really fall into either category in a convincing manner. Many are used in compression fittings, where ODs are the sealing surface, and others are used on barbs where the ID is the sealing surface. You really have to do your homework, and make sure the sealing surfaces of the tube or hose or pipe have proper fit.