The manual states:

"First basemen's defensive ratings. This is another nightmare as far as statistical
analysis is concerned. STATS, Inc. gives us ZONE ratings and we factor in assists
per inning adjusted by pitching staff factors (gb/fb rating and staff strike outs), but
there is no way to judge their ability to dig out bad throws."

It looks to me that the assists per inning is the major factor though. Check out these stats:

INN A A/INN PB Range
Mientkiewitz 1053 56 .053 0.6
Konerko 944 60 .063 0.9 (something else or PB error?)
Thome 982 61 .062 2.0
Stevens 1011 72 .071 3.0
Grace 849 47 .055 4.0 (reputation?)
Snow 554 37 .066 6.0
Helton 984 84 .085 8.0
McGriff 707 70 .099 9.2
Bagwell 1088 114 1.04 9.9

I won't guarantee I didn't make any errors with these numbers, but hopefully that is true. It looks like a pretty highly weighted stat to me. I'm not sure I'd have put quite that much emphasis on it though.
At the least though, it does look that a pretty subjective measure is used and not just "reputation". I like that personally.

It also explains why Mientkeiwitz has a lousy range rating - he deserves it.

DaveF