First of all, you will have drive train losses, so it is to be expected that your Elite measures lower power numbers than your Stages. Depending on how well you are maintaining your drive train, you should expect about a 5 % loss in your drive train. If you haven’t oiled or waxed your chain in while, this could be higher. So at 250 W that alone is about 12.5 W.
Then your Stages is left-only, so if you have an imbalance of, say, 2 % (51-to-49) and your left leg is weaker, this gives you another 5 W. So just taking these two factors into account, we are already at 15-20 W (if you are doing 250 W).
Another point is accuracy. Elite’s power meters are quite famous for their accuracy. In their high-end trainers, they are the best on the market. Nevertheless, you have an error that is typically 1.5-2 % for each power meter. You add that on top and you easily can see that this may account for another few watts.
So I think everything is working properly in your case. If you want to slightly inflate your power numbers in Zwift races, I guess you can choose to believe your Stages power meter. But for training, I’d probably stick to the Elite since it’ll allow you to do single-leg drills. (I have a left-only power meter, too, and single-leg drills get a bit wonky — I just ignore power numbers and look for consistency.)