Why? Have you seen every one? Even if you had, is it the standard because they say it's the standard or do they say it's the standard because it is the standard? If the former, what is their authority to proclaim this standard. If the latter, just who is the authority that proclaims the standard?
Bob...I was actually thinking about you yesterday, thinking that I haven't seen any responses lately and hoping all was well with you, so good to see you here.
To answer your questions...
No, I haven't seen every catalog but all those that I have seen use the same measurement convention.
I stated it to be the standard because I declared it the standard! Do I have that authority? Depends who you talk to I guess!
Seriously though, I don't believe that any real standard exists, however from my experience, I do believe that the WxH convention is the typical in the sign industry, though I understand that it is not always comfortable for everyone and it is highly debatable. I also agree that to hold to either belief and assume that anyone will understand you will not work, it almost always requires clarification. Furthermore....as Gino declares, does it really matter? I doubt it but it makes for great conversation here!