I'm probably comparing rotten apples with putrid oranges (and I don't know my wattage from my frottage) but, FWIW, I've now ridden the 7 day free trial with my Garmin* running too and ended up with..
Garmin 164.1miles @ 20.1mph
Zwift 167.8miles @ 20.5mph
... BUT I think that only happened to be close because I targeted 4 hilly rides as the speeds seemed so crazy on the flat ones - and the extra mileage from those got reclaimed in the hills. Needless to say, I've never ridden the perfect bike, in the perfect position, on perfect roads, in a skinsuit, with a boatload of drafting either (and I guess the algo has to accommodate those guys too) so I don't know what sort of theoretical advantage you'd expect from that, but I suppose this is how they convince people to drop on £1000 proper trainers to find out.. sigh.
*I have no idea if the Garmin x myTurbo is even slightly accurate either but I at least know it makes me want to puke if I go much faster than that ^ (and in that respect it's just like real life.. sob..)