We pronounce the letter "W" as double-you. As phonetic shortening of "Double" gives you "Dub" so J-Dub is a phonetic shortening of JW even though there are more characters used. In other words, the oral shorthand become the longhand on keyboard. But the familiarity of the oral shorthand carries it into cyberspace anyway.
Hope that clears up the mystery for you...I have always thought it was fitting another reason, "dub" is an English word that can mean "make a copy of" and that is the net effect of the J-Dub teachings.
Respectfully,
OldSoul