Dave,

The odd number such as 99 instead of 100, helps operators in the contest 
know if they worked someone with that exchange without having to inter it in 
the log to see if you worked that station already or not.  Years ago, a big 
contester was KH6IJ.  He used to send 5nn ttt.  The tripple T stood for 1000 
watts or, obviously a KW.  As I comb the band, the exchange often allows me 
to stay on frequency to work the guy or to skip working him because I 
recalled his unusual exchange.  The Japanese also use 5nn u5 for 599 25 
which is their zone during CQ Worldwide.  I've also heard DX contesters 
using tnn nnn for 599 and 999 watts, or in other words, a kilowatt.  So it 
is just a way of keeping the exchange short and sweet.

Phil.
K0NX