There the same lens with the name written differently. Most times this is a simple mistake when transcribing a name from the manufacturer. Lens names are complex and indeed look quite similar and honestly, Tokina has one of the more complicated schemes which they use inconsistently.

As I have a site about digital cameras and lenses, Tokina sends me this information with each new lens. There were only two 11-20mm lenses produced for Canon EF mount and theose are the AT-X 11-20mm F/2.8 PRO DX and ATX-I 11-20mm F/2.8 CF. These lenses have a different finish but otherwise almost identical. You can compare these lenses side-by-side here on my site Neocamera.

ATX-120 and AT-X 11-20mm F/2.8 PRO DX refers to the same lens. The former is the model number and the latter is the model name. The second name you found is a combination of both, so looks like a data entry issue and not a different lenses.

Interestingly, you can save a lot of money by buying incorrectly named products! Stores respond to low sales by lowering prices and if something is incorrectly named, then people don't find it, so there are fewer buyers. I vaguely remember someone make a ton of money from buying incorrectly listed designer clothing and selling them back with the correct name!