It has nothing to do with discrimination based on ethnic or national origin, color, race, religion, or sex [or residence of anyone].

All USSC case names are styled “Petitioner v. Respondent”, regardless of which party initiated the lawsuit in the lower court. The party that lost in the lower court is called the petitioner, and the party that prevailed is called the respondent.

DC lost in the lower court (DC becomes the petitioner).
Heller was the only one with “standing” in Parker et al. v DC (Heller becomes the respondent).

Therefore, if the USSC grants the petition for writ of certiorari (Cert.) the name should be “District of Columbia v. Heller et al.”

All papers submitted to the USSC will refer to the case as “Parker et al. v DC” UNTIL the USSC grants cert–[IF cert is granted] then it becomes a new case with a new name.

For the whole reasoning of why only Heller had standing, see the published decision in Parker v DC [PDF file], pages 5 through about 12.
http://tinyurl.com/ywcg7k

The the gist of it is:

“The noteworthy distinction in this case—a distinction
mentioned in appellants’ complaint and pressed by them on
appeal—is that appellant Heller has applied for and been denied
a registration certificate to own a handgun, a fact not present in
Seegars. The denial of the gun license is significant; it
constitutes an injury independent of the District’s prospective
enforcement of its gun laws, and an injury to which the stringent
requirements for pre-enforcement standing under Navegar and
Seegars would not apply. Since D.C. Code § 22-4504
(prohibition against carrying a pistol without a license) and D.C.
Code § 7-2507.02 (disassembly/trigger lock requirement) would
amount to further conditions on the certificate Heller desires,
Heller’s standing to pursue the license denial would subsume
these other claims too.”

[...]

“In sum, we conclude that Heller has standing to raise his
§ 1983 challenge to specific provisions of the District’s gun
control laws.”

I haven’t researched it myself, so I won’t claim to be absolutely for or against his claim, but it sounds…off. I wouldn’t give it much merit, partially because I also heard that standing was the real issue. If he has proof, that’s great. But an anonymous insider isn’t cutting it for me on legal issues.