This depends on the available technology and hardware that they have setup. The network speeds can vary depending on what type of wires the ISP has connected and what tier 1 internet service provider they are using.
The speeds you are getting are simply limited bandwidth amounts of the networks actual capable speed. Each individual price plan takes off some of that limit. This is why you will notice speed variations throughout the day during peak network usage times. More people are using the network so there is less bandwidth available for you to use.