Different Reading Zwift and Garmin

I have PowerTap’s Power Pedals. Before I ride indoors, I first pair them with Zwift. After that, I pair them with either my Garmin computer or watch. Zwift reads exactly the same cadence but about 10% higher wattage. Both are set to display Avg 3 sec power. Since both readings are coming from the same source, any idea of why there would be a difference? Maybe Zwift only picks up one pedal?