FWIW, I've done long, long periods of all trainer training as well as purely outdoors. I find that the longer, more often I'm on the trainer, the more cadence drops. A general estimate would be a drop in cadence of about 10-15 rpm for the very same sessions. For example, I've seen an avg cadence of < 70 rpms during some grueling 2 x 20 indoor sessions but would never experience that outdoors. I've seen a lot of anecdotal evidence of many others experiencing very similar patterns. Yet once I start riding outdoors mostly, the trend switches to higher cadence.
Personally I don't care about cadence for the most part, especially in an off-season build period, and focus on power.