Dont worry. Now I get you! My RDA hole is 12,8mm. Glasadapter 14mm end - on the tip: Mine is 12,1mm.
In my setup I can stick the adapter at least 5mm into the rda. My phobia original drip tip is 12,5mm too.
Hmm... so I have just been "lucky" ? Sorry... I was just LEGOing on and it fitted in my...