I won't get into the why but let's take Netflix and 4K movies as an example. You only need 25 megabits to play a 4K UHD movie and the iphone, ipad, apple tv are just players so this is fine.
As to high speed WiFi to hit those speeds all parts must work together and as there are many versions of WiFi radios some don't hit the max speeds. On top of that you see the speed taper off as you move from under 3 meters to further away. This is not a bug.
I have an internet speed of 100mbps thru my cable companies modem. I have my Mac connected to the modem as well as my Netgear Nighthawk router. When I run a speed test the Mac easily exceeds the 100mbps but my iPhone, IPad and Apple TV can do no more than 16mbps and I have the Apple TV hard wired to the router. I’ve even tried connecting the Apple TV direct to the modem and I only get 16 and the connection light on the modem is orange instead of green. I do have a long cable run from the modem to the Apple TV. Regardless of connecting directly, shouldn’t the router deliver 100mbps wirelessly? Thanks