Broadband speeds can fluctuate for various reasons. Subsequently, Internet Service Providers (ISPs) advertise broadband speed as "up to" a certain amount, as that's what they're targetting. Most ISPs offer a minimum service level agreement (SLA) detailing the minimum speeds they will aim to provide you with. If they cannot provide these speeds, you may be able to end your contract early and move to a different plan or find an alternative ISP.