Which trainer?

I feel your pain. People buy power meters to resolve uncertainty, not to increase uncertainty. Many people are convinced that accuracy and the ability to check it is unimportant -- they're convinced that consistency is all that matters. Here's a case where your Stages appears to be consistently lower than your trainer but you don't know which is right, which is causing you more uncertainty about your training.

Many (but not all) power meters allow you to check their accuracy fairly easily. Sadly, the Stages isn't one of them, so you don't know whether the problem is you, your trainer, or your Stages. There are ways to check the Stages but they're considerably more trouble -- or they require a calibration rig or another power meter that is known to be accurate.

Fans of the Stages often point to Team Sky and say if it's good enough for Sky it's good enough for them. Team Sky has enough other equipment to check their Stages. Most people don't. If you really want to know, you'll have to find someone with a known accurate power meter who is willing to help you determine whether it's the Stages or your trainer. If both are okay, then it's you: you have a huge left-right discrepancy.