T5 merely designates the nominal diameter of the bulb - 5/12" in this case - and that determines the pin spacing, too. (T12=12/12", T8=8/12", but they have the same pin dimensions, as the T8 is a more efficient retrofit.)
In the standard "four-foot" (actually about 46") bulb, a T5 is a 28 watt lamp, while a T5HO is 54 watts, so puts out a LOT more light.