Pretty sure a 1080Ti doesn't have 3GB.
I love this application but this one little thing keeps bugging me. C'mon devs I know you can fix this.
I don't suspect they can.
At least, not easily.
The size of the Speccy executable would indicate the software does little heavy lifting of its own so must rely on system DLL's and WMI requests.
A quick search of the Speccy forum will show many users with similar wrongly reported memory issues, both with motherboard and video card memory, spanning a long period.
Have you started an elevated command prompt, typed in WMIC and interrogated what WMI thinks your video memory is?