My guess is that Grizzly assigns full p/ns to their parts because they are setup for replacements parts sales on their website. Harbor Freight does not but they aren't the only ones.
I've never actually completed the order for a replacement part from HF cause the part I wanted was NLA. I hear HF is pretty good at supplying replacement parts, but often they are not in stock so you have to wait from them to arrive from overseas but they do fullfill their orders.
When calling HF for a replacement part, they will ask you for the item # of the machine, then ask you for the replacement part number listed in the manual. Then they will tell you pricing & availability.
The simple 2-3 digit number of the replacement part in the manual is typical of Asian import machinery, very basic & seems generic, often because it is, the same manual might be provided for lots of machines with different brand names.
When I ordered replacement parts from Enco for my old Enco bandsaw, it was the same method as HF. Enco asked for the bandsaw p/n, then asked for the item number for the replacement part I needed. The manual for my Enco bandsaw was just a Rong Fu manual & it also used the 3 digit numbers.
But with Enco they had internal p/ns assigned to the replacement parts I needed. Those p/ns were only available over the phone once you went through the process of telling them the part(s) you need. They were happy to give out those internal p/ns, not like it mattered since they were theirs. With Enco you couldn't order replacement parts online either, even if you tried to enter in their internal p/n. I suspect HF probably does something similar with internal p/ns for their replacement parts.