Peering between your ISP and Valve is likely saturated.
Considering Valve has an incentive to make downloads fast (= more revenue), it's likely your ISP is being stingy in this case.
And ISPs in most of the western world have no incentives to fix it, instead trying to scam Valve and others to get paid twice.
I would have assumed valve use some third party CDN but yeah this would make sense I guess