Example test below [0], admittedly the downstream results aren't the best but I've got some rather large downloads and streaming going on right now. Upstream shows the capability though.
I must admit I'd probably not upgrade anyway though. 100mbps is enough to carry a couple 4K video streams in parallel. So not going to outgrow that speed anytime soon.
I attribute it to less dropped packets, less interference, and full duplex, despite lower bandwidth.
Don't forget that Wifi needs to be properly positioned to work quickly.
You lose packets to radio interference (or collisions). And SMB in particular reacts poorly to packet loss.
You're also assuming that "good" wifi deployment is a reasonable expectation. People just buy that blue Linksys thing and put it near the jack for the modem. If you're going to run wire all over the place for access points then you might as well run wire all over the place for ethernet.
And if my properly positioned you mean "In a faraday cage with only the AP and your station", you'd be correct. The issue is real world WiFi doesn't ever work that way. There are very few places where you don't pick up at least one or two neighbors using at least some of the channel bandwidth you need. Even inside your own wireless domain you have the hidden node problem, forcing wireless domains to be broken into small cells using low power greatly increasing the costs of deployment. Add that a huge number of cheap APs and computer drivers flat out suck and the average non-professionally installed wireless user is not going to see 50% of the bandwidth they should.