Honestly I don't know and I'm not going to come here and pretend to talk about stuff I don't know.
I know as much as the next guy and all I'm doing is providing information readily available to everyone else respectfully just like the rest of y'all.
My best educated guess is they tagged them 15 ohms because that's what they are rated for production on the UK Greenbacks like the EVH/Heritage 20 watts vs the Chinese RI 25 watts at which isn't much on my side other than stating the obvious respectfully. No real big mystery there.
Also you mentioned something about 12 ohms earlier, look at the spec sheet and look at the DC Resistance, not entirely sure if that is what you meant but yeah. It reads 12.3 ohms per the spec sheet straight from Celestions website.
Im not doing anything else here other than presenting actual data, facts and information available to absolutely everyone here and that is it. No harm, no foul