No way to give a real answer. Could be the site you are downloading it from either having a capacity issue or they could intentionally be rate limiting. You ISP could also be rate limiting your traffic. If they were to say limit you to 1m/second they could either let you send 1m every second or they could let you send 10m for 1 second and then 0m for 9 seconds and get the same average.
The only way you could really test any of this is if you can control both ends and send traffic at defined rates and measure throughput. IPERF is line mode tool that will do this but you will need someone at another location to work with.