It’s not unusual to see differences between on-bike power and smart trainer power numbers.  The difference can be for myriad reasons including things like the type of power meter (pedal/crank arm/spider), the calibration of the unit, and the type of smart trainer you have (wheel drive/direct drive).  

Although Zwift will pull the numbers from the trainer, you would be better off using the numbers from your pedals as they are likely to be accurate as well as the ones you would want to base your outdoor workouts on.