Your cadence says nothing of the gear you were in. 90 RPM at 80GI is faster than 90 RPM at 70GI.
If you kept a similar average cadence, that means you were in a shorter average gear. It's math.
I assumed he knew this and wondered if he was matching his regular gear inches with the corresponding gears on the dérailleur bike.
Even I haven't done this with my Bianchi. I don't know what gear matches my T1 and its 81gi.