Why does Linux Mint show less RAM than Windows?

The two screens may not be counting the same thing. Some RAM can be reserved for hardware or firmware, including integrated graphics if your machine uses shared graphics memory. Linux may exclude some reserved memory from the total it reports.

Check the BIOS or firmware settings for shared graphics memory, and compare Windows' installed RAM and hardware-reserved figures with Linux's reported total. That should give you a better clue than comparing the two headline numbers alone.

Check the labels, but don't assume this is just GB versus GiB. A GiB is 1,073,741,824 bytes, while a decimal GB is 1,000,000,000 bytes. Windows often labels binary memory figures as GB, so the labels alone don't prove the units differ.

For scale, 32 GiB is about 34.4 decimal GB. The difference between 31.8 and 31.1 is much smaller than a full GB to GiB conversion would suggest.

Also make sure you're comparing total memory with total memory, not total with available or free. Linux's available figure changes as programs run and memory is used for things like file caching. That is separate from the 31.1 GiB total you're seeing.

In Mint, you can run free -h in a terminal to see the total and available figures side by side. Those numbers won't by themselves identify what reserved the missing memory, though.