The AN (for Army Navy) standards as noted above can run small.
Hard line tubing is spec'ed as an O.D. size plus a wall thickness. For the hardline components to interchange they must fit a common size, and that's the O.D. Different wall thicknesses are used for different pressure requirements. So, a 3/8" (-6) hard line with a 0.020" wall will have an I.D. of 0.330", or just larger than 5/16".
Flexible lines are spec'ed by their inside diameter and run close their respective sizes. The fittings are specific to the hose manufacturer (or should be). In most cases the fittings have a much smaller bore than the hose they are attached to. The Aeroquip hose on my car that's used to tie the primary and secondary bowls together is a -6 line but the bores in the end fittings is 1/4"!