That last sentence is the key point here. No ISP can really guarantee a speed you get because
1) they don't own the entire internet, only their portion of it
2) ultimately, the bandwidth is shared at some point. If 1000 people in Virginia Beach buy a "guaranteed 25Mbps" and try to use it simultaneously, they'll saturate the ISP's uplink, which is almost certainly not 25Gbps.
Contracts (they're called service level agreements, or SLAs) that do truly provide guaranteed...