[RTTY] Unshifted Numbers

[NB1B]

Being a relative newbie to RTTY contesting, I made (for me) a startling 
discovery this weekend.

After continually receiving PPQ in response to my "NR? NR?" inquiry, I 
discovered that "PPQ" is the same as "001".   After operating for 24 hours, 
it seems like the following letters equal the numbers:
P=0
Q=1
W=2
???=3 (never did figure that one out)
R=4
T=5
Y=6
U=7
I=8
O=9

That "TOO" I see on my screen didn't mean that I also got a "599"- it was 
the "599".

What mechanism makes some programs send the letters instead of the numbers? 
I don't think the problem was on my rx end when someone would send me "TOO 
PPQ PPQ", I'd ask for the number, and he would repeat the same string of 
letters a second time.   What actually clued me in to this was a station 
would send "599 PQP 010", so I'd ask for the number, and he'd send "599 010 
PQP" back.

Dennis NB1B