Speeds following plan change

Interesting find. I recently upgraded my double play plan, with internet going from 300/10 to 600/15. When I check speeds now using The Speedtest app, I only get that new download speed if I manually choose the Comcast server. Every other server only shows 300 mbps download (which they showed prior to my upgrade) All show the new 15 mbps upload speed however. Is Comcast using some sort of hocus pocus here? Prior to my upgrade, they all showed relatively the same speeds.