More than likely its a lower gear ratio in the trans.
Looked through a few older dealer catalogues but they were vague on what is included with a Trailer Towing Package and with most car being Front Wheel Drive its not even mentioned or offered. The 91 Caddy book only indicated a TTP on the rear wheel drive Brougham and not the FWD DeVille's.
It consisted of the 5.7 V8 , 124 Amp Altenator and wiring harness. No mention of heavy duty shocks / suspension or additional trans cooler..
========= Harryface =========
1991 Pontiac Bonneville LE ~_~_~261,000 miles_~_~~_