Both the cranksets that were linked in the original post are identical FC-RED-D1 cranksets, with the cheaper crank being an OE unit and the other being a standard retail crank (as indicated by the information disclaimer on the product page). OE units are typically cheaper and available only as manufacturer/assembly overstock, so they don’t usually have the same SKU availability.
For a crash course on model number logic, this article: Model Code Info | SRAM
is helpful. In the case of “DX” suffixes, the letter indicates the numeric generation (D = 4th) and the number X is the iteration of that part, within the generation (1 is the first and only in the case of the RED product series, whereas Force has both a 1 and 2 iteration, with the latest release).
Ultimately, if the OE unit has availability in the fit spec you’re looking for, I would pick it up at a good price. All the OE units that I have ever sourced from places other than SRAM for my own builds and customer builds have always been great!