It's an interesting question. I also joined in '89, yet I do not appear to have the discrepancy. My summary reads as follows (you will note that my "ending balance" matches my "miles with expiration deferred" and "total available award mileage"):
Mileage Summary Summary Date: Feb 2004
Program to Date Miles: 2,392,283
YTD Elite Qualifying Points: 22,460
YTD Elite Qualifying Miles: 19,445
YTD Gold / Platinum Qualifying Segments: 7
Miles Towards Gold / Platinum / Executive Platinum Upgrades: 59,058
Miles with Expiration Deferred Through 2/06/07: 587,283
Upgrade Account Balance: 11
Total Available Award Mileage + 587,283
+ Flights, awards, and other eligible transactions processed within the last two weeks may not be reflected.
STARTING BALANCE 560,591
01/29/04 TRIPLE MILE PROMOTION 0 5,222 5,222
01/29/04 AA.COM BOOKING BONUS 0 500 500
01/29/04 AMERICAN AIRLINES 222 Y LAX BOS 2,611 2,611 5,222
01/28/04 AA.COM BOOKING BONUS 0 500 500
01/28/04 AMERICAN AIRLINES 29 Y JFK LAX 2,475 2,475 4,950
01/27/04 AA.COM BOOKING BONUS 0 500 500
01/27/04 AMERICAN AIRLINES 144 Y LAX IAD 2,288 2,288 4,576
01/27/04 AMERICAN AIRLINES 25 Y BOS LAX 2,611 2,611 5,222
TOTAL MILES POSTED 9,985 16,707 26,692
ENDING BALANCE 587,283