You don't say what percentage of the Stages power that "40 to 70 watts" represents.
What were the wattage numbers?
What trainer model is this?
Fluid Trainers
Fluid trainers are supposed to have a predictable power curve, based on wheel speed. So, Trainer Road uses that curve to calculate power.
So if the standard power curve isn't exactly correct for your trainer, the calculated watts could be off. I've heard that fluid trainers have different resistance once they are warmed up.
Tire inflation can affect the power due to tire flexing losses, but then I'd expect the Stages to be higher.
Kurt Kinetic power calc
See their page
discussing their power calculations.
From that page:
Limitations
While training with speed-based power is affordable and accurate and works amazingly well for longer intervals and overall ride averages, it's not strain-gauge based and does not measure torque, so there are a few things we cannot do. Unlike much more expensive systems, the Kinetic inRide will not register a spike in wattage from a sudden acceleration. The wheel must turn faster before a higher wattage reading is observed, so readings are not instantaneous.