2 things here:
You either have the internet modem set to 100mb and the card is auto-negotiating to that 100mb speed or
You have a hub that is 100g in between your computer and internet modem and it is syncing to that speed.
Make sure everything is on auto-negotiation settings and it should provide maximum capabilities of the device.
InWin 909 Silver RGB build
i9-10900k
ROG Maximus XII Apex
ROG Matrix 2080 ti
Gskill Trident Royal 4400 16g
Everything overclocked and love it.
ROG Awesomeness !!!!