Ah, so it's "initial window size" than than "ping round trip". Interesting. The RFC they reference doesn't seem to be elevated to official standard, and it appears different OSes have different default values.

On my Mac sysctl net.inet.tcp.maxseg_unacked says 8, which I assume is the relevant TCP option. But in the case of a server responding to a HTTP request I suppose the server could set it to basically any value, so why stop at 10? :)