Association rules - ARRAY vs. TIDList

Dear KNIMErs,

I've encountered some strange behaviour in the association rules node - choosing the default "Array" on a 200k records, 11 items list would give me ~ 4k item sets (with some spurious "item11" label not contained in the original data set - probably and implementation glitch), whereas "TIDlist" would only give me around 2k (and not the spurious item, which was infrequent to begin with).

Any pointers as to what could be causing this drastically different result with otherwise unchanged settings?

Thanks
E