Its not just your router, also intermediate hops. I saw this same thing back when 1gbps was first being rolled out 20 years ago. My downloads would hard lock at 100mbps from a lot of sites while other locations would pull at the full gbps speed. Turns out some router in the middle would max out at a much lower limit. OPs test is showing the same thing. FWIW, Steam and Battle.net use P2P to distribute data so you get multiple streams using different routes. But GitHub, as example, has a single route between their datacenter and you and goes through a bunch of routing shenanigans once you hit their POP.
Get ready for a whirlwind of delayed network improvements from your favorite transit provider for the next 3-5 years just as we saw in the early 2000s!