No its not at all. I think you're e-statting.
Agreed.
18" is definitely not a 'regular' sized arm and is considered by most very hard to achieve. Put size onto arms takes a very long time they don't respond like other muscles such as the pectorals, lats or quads. BIC3P and FF have those size arms and they are monsters. I've tipped 17" but that was whilst on C-mono so had some water retention.

).

, it may just take a bit longer the older you get
I'm uber jealous (though I don't train arms that hard so it's no surprise). I guess you might just be genetically lucky to have big arms and a small chest. Perfectly feasible, just lucky genetics! 