Broadband speed can be affected by the number of devices connected, where your router is, and even the time of day.
The speed you see when you order a broadband service is usually the maximum possible, rather than the speed you'll get all the time. That's what ads always say things like "Up to..." etc.
The speed you'll get day to day is affected by:
- Whether you're connected to fibre cables
- The condition of the cables in your home
- Distance from your home to the broadband exchange (for some kinds of broadband)
- How thick your walls are
- The distance from your devices to the router
- The quality of your broadband router