Diff.tbl is a wholly unofficial hack of Ckit.exe, the FRUA program. Diff.tbl must be applied by UAShell before it has any effect.
Thus there would seem to be only two possible problems: the file itself or the UAShell and DosBox. You could try running the UAShell without DosBox, that will tell you if the Box is at fault.
In the Hacking UA forum you can read about the problems people have trying to get the UAShell to even generate a Diff.tbl when it is run from the Box!
Check that running UAShell through DosBox has not messed up the Diff.tbl file...