My goodness, just print my stuff

I bought new ink cartridges for the printer yesterday. The helpful indicators said that my cyan, yellow and magenta were low. This should not be a big deal, I thought, because I only had to print something in black. But actually, no. My fancy, awesome printer won't print in any color if any other color is deemed too low. So, off to Target I went to pick up my colored cartridges.
The three colors were there, but of course, the black was missing. I figured that, as a smart person, I should pick up a black cartridge too, because that would definitely probably go out before I finished printing what I had to print. This is where it gets tricky, though. There was no 932 cartridge, but there was a 932 XL cartridge. I had no idea what to do with that, so I asked the friendly and "helpful" Target electronics-section employee. The guy was afraid to leave his counter area, so I had to ask his opinion over it, and he did his best to pantomime what he thought was a proper answer. His professional opinion was that, if I didn't already know what the XL meant, I should not buy it. And maybe try Staples.
Staples was infinitely more helpful. That store also only had the XL cartridges, but there I also got a decent explanation. If my printer accepted the 932 cartridge, it would take the 932 XL cartridge. It was double the size, but there was a dedicated spot for it in the printer. The XL has more ink than the regular ones, and costs more, of course. My question: Why would they have an "XL" cartridge, rather than just have the regular cartridge bigger? Why make the differentiation? Also, why not educate the dorks in all the printer areas so they know this, too? It would have been awesome to not have had to drive to Staples, where the cartridge cost $3 more, as well.