10% is the total return, but you see 6-7% because that's the real growth after inflation, which assumes average inflation of around 2-4%.

So you're getting 10%, but in terms of actual, real, concrete gains it's more like 7%. Also remember that's the average so it's possible to see years of 5% and then a few of 14%.

The 7% number also looms large because at 7% a year you double your money in ~10.3 years; makes it easy to plan for 30+ year investment horizons.