What's the distance between the WiFi Router / Modem and the device connecting? (Desktop, Laptop, Mobile etc) Is there anything in between such as walls? All of these can make a fair impact
As others have said, the 100Mbps is the link speed between your device and the Router / Modem.
Example connection;
Website ------- 20Mbps -------- Internet ------- 10Mbps ------ Your Router ------- 100Mbps ------ Your Device
The fastest connection speed available to any point is the slowest of all the routes along the way. So you can see that the even if your Device is reporting 100Mbps, you are never going to get that if your connection to the internet is slower.