Unlike our Quarq power meters, your gen 3 Stages and the Garmin pedals do not estimate left and right power, they measure both separately. So the latter do not use any L/R estimation algorithm. Of course, it doesn’t explain away discrepancies between all these power meters.
But if you made it your goal to stay within a certain L/R balance window, say, 48/52–52/48, then you base your L/R power balance off of one single source of power, trusting that this number is accurate. In my experience, though, simply displaying L/R balance and wanting to stay away from more extreme splits is enough to. (I noticed that after switching from a one-sided 4iiii power meter, I had developed a significant imbalance — at least if my Quarq is to be believed.) Point being, you don’t necessarily know whether 50/50 actually is 50/50.
The Stages has to deal with the Shimano crankset, so I don’t know whether I’d put more stock in those numbers than the Garmin pedals … 
Deep diving on power meter data is a big mess. (When I found out that my Elite Suito and my Quarq DZero started disagreeing significantly about how much power I was putting out, it really triggered my OCD. Eventually, I decided to simply declare my Quarq as the “source of truth”, not least because I was relying on my Quarq when riding outdoors.