Why don't the readings from my power meter and trainer match?

Exertion is different for everyone and you're obviously comfortable with how certain power levels feel to you.  That said, it's quite common for a power meter on the bike and the power reading from a trainer display different output.  

There are a number of factors that go into this but in the simplest terms possible, the power reading is likely accurate, within the respective manufacturer’s +/- range, where it’s being measured.  Meaning the power level read at a set of pedals will inherently be different from the power level something like a hub, or trainer reads.  There is also the +/- difference in range to consider for each individual meter – if they’re each to opposite ends of their tolerance spectrum, that will widen the gap between the readings.

Drivetrain loss/variance is also something to consider, especially when comparing readings to something like a wheel-on trainer like the M2.  Power travels through the drivetrain, through the spokes, then the tire, then finally onto the roller on the trainer.  

Interference can also contribute - if multiple power meters are in close proximity, there can and will be some competing data that is being transmitted.  We recommend pairing the trainer to the Saris Utility app (iOS/Android), performing a calibration, and then an ERG workout with no other power meters nearby or transmitting to verify results.  This can be built off of as a baseline.