Some items to think about are accuracy of the measurement, pre-existing injuries, the fit versus any asymmetry in your body, the fit versus how you actually pedal, and
leg/hip strength imbalances.
As said above is that an accurate measurement? Can you jump on a Wattbike at a local gym and see if you get similar results? Is that a constant imbalance or only at particular power/cadence? Has the power meter got the latest firmware? Was it installed correctly?
Have a think about any injuries anywhere in the chain from back all the way down to ankles, is there anything that might need addressing properly? There could be a lack of activation on one side for example.
Is your fit suitable? Could you be physically or functionally assymetrical? Perhaps one leg is different in length to another. Further to this some people get a bike fit when fresh and then the slightest fatigue ruins their form.
Can you try (under instruction if necessary) some strength benchmarking using both legs and then single legs? That would highlight strength imbalances and could even be as simple to start with as single leg squats to failure (with good form!).