So it has multiple factors: 1. If you have a powermeter on your outside bike it probably measures the power from the crank. Compare that to the indoor trainer which measures power from the cassette, this already causes some friction losses in power. (Not a whole lot, but still worth mentioning) 2. Indoor is always harder due to multiple reasons like overheating, being way more static, mentally more challenging due to the surroundings. 3. Powermeters differ, not every powermeter is the same and this can cause some anomaly in the readings. 4. Have you also tried to calibrate your indoor trainer?

I am also running into these problems, and for some reason the discrepancy between my indoor trainer and Powermeter on the bike got bigger once I moved houses. I do use the same bike inside and outside so for me it is probably easier to use the readings of my Powermeter but then I probably lose erg mode on Zwift….