To get an accurate reading you will need to plug a laptop directly into the router. Testing over WiFi can give lower results.

When a WiFi router detects other devices broadcasting a WiFi signal it will back off its data rate performance to ensure Internet packets are delivered and not dropped. When you perform a test it is common for the router to drop back on its data rates and you will see a lower result. This is not always a fault with your connection, it is a mechanism to ensure your Internet packets are delivered reliably.

This is common when using the 2.4Ghz radio channel as most wireless devices use this, Microwaves, baby monitors, video senders.