Doh!
I should have read the entire thread that James' message was in. At the end of it all, Bill Butler comes to the rescue and explains the results in terms of integer arithmatic and rounding in armour damage.
POSSIBLE SOLUTION
using integer math
Let Y(0)=1000 and k=10
Y(n) = Y(n-1)*9/10 , X(n)=Y(n)/k
n Y X
1 900 90
2 810 81
3 729 72
4 656 65
5 590 59
6 531 53
7 477 47
8 429 42
9 386 38
10 347 34
11 312 31
12 280 28
13 252 25
14 226 22
15 203 20
16 182 18
17 163 16
18 146 14
19 131 13
20 117 11
21 105 10
22 94 9
23 84 8
24 75 7
25 67 6
26 60 6
27 54 5
28 48 4
29 43 4
30 38 3
31 34 3
32 30 3
33 27 2