Why do VOR A, and B charts only have circle-to-land minimums, while VOR X, Y, and Z charts have straight approach minimums too? Is this the only difference and is it only used to separate the two chart types?
Yes, that is the difference. From the FAA's Instrument Procedures Handbook, Chapter 4:
When two or more straight-in approaches with the same type of guidance exist for a runway, a letter suffix is added to the title of the approach so that it can be more easily identified. These approach charts start with the letter Z and continue in reverse alphabetical order. For example, consider the (RNAV) (GPS) Z RWY 13C and RNAV (RNP) Y RWY 13C approaches at Chicago Midway International Airport.
Approaches that do not have straight-in landing minimums are identified by the type of approach followed by a letter. [...] As can be seen from the example, the first approach of this type created at the airport is labeled with the letter A, and the lettering continues in alphabetical order.
In actuality the circling approaches are named such that the suffix is unique across all airports with the same name within one state, and unique across all airports serving any one city, as laid out in this Av.SE answer.