Bandwidth is shared.
When several devices use the connection together, they share the available capacity. A household streaming, uploading and joining video calls at once has different needs from someone browsing alone.
Downloads and uploads do different jobs.
Downloads bring content to you. Uploads send your files, camera feed and backups out to the internet. If you often send large files or use video meetings, ask about upload speeds as well.
Latency matters too.
Latency is the time data takes to make a trip across a network. It can affect real-time activities like calls and games. More Mbps alone does not guarantee lower latency.
Test with a little context.
Pause large background transfers and use a wired connection where possible when assessing your line. Your device, Wi-Fi, the destination server and time of day can all influence the result. Avoid judging a package by a single reading.