Trevor Wilson 1993 has OAV+ of 96 a BB/9+ of 96 but a WHIP+ of 100.

IS that possible? or is there an error?

yeah, easily plausible. others are 97/90 or something like that.

If WIS had a stat that was hits/9+ to go with BB/9+ you would get the relationship you're expecting with WHIP+ (walks + hits per inning). But OAV is a batting average, not a hits per inning measure. Averages like OAV will be effected by double plays, errors, sac bunts, etc. So you can't combine OAV+ and BB/9+ and expect it to be precisely related to WHIP+.