FN is a large manufacturer of firearms from handguns to heavy machineguns and has customers worldwide.
There is no universal law decreeing where a firearm's serial number must be. In the US, the frame is the firearm and must be serial numbered. In South Africa, it is the barrel. You get the idea: FN must produce firearms meeting legal requirements of their individual customers all over the world.. There may be countries in which it is required that all three major components of the Hi Power have serial numbers, but I do not know that for a fact. I suspect also that having the barrels, slides and frames of individual Hi Powers numbered make it easier for military units maintaining multiple Hi Powers keep the pistols assembled as they came from the factory after routine field-stripping, cleaning, etc.
Some of my Browning-marked Hi Powers came with the serial numbers only on the frame while others bear them on both the frame and barrel. I have seen them marked on the slide, barrel and frame as well. There doesn't seem to be much rhyme or reason to why unless from possible military overruns or pistols initially intended for purchase in countries other than the US.
The major difference in serial number locations I've seen in more recent times involves the frame. On Browning-marked pistols, it remains on the front grip strap while FN-marked Hi Powers are serial numbered on the side of the frame.