[CQ-Contest] Why send 5nn ?

[Ktfrog007 at aol.com]

Some people send ENN.  I hear ENN a few times in every CW  contest.  I 
don't think I've ever heard E for 5 in a serial number,  however.  I hope I 
never do.
 
73,
Ken, AB1J
 
 
In a message dated 2015-07-30 3:34:32 P.M. Coordinated Universal Tim,  
jpklemola at gmail.com writes:

Why  people send abbreviated 9 but 5 is not abbreviated?
I am not Vulcan, but  there is no logic.

73,
Jukka  OH6LI