Chris Chau's questions

I'm using raspberry pi 3B+,ubuntu 20.04 pre-installed server,download from here:

https://ubuntu.com/download/raspberry-pi

I've found that if I use free command (whatever -k -h option),it show me I only have 852MB Ram

root@ubuntu:/home/ubuntu# free -h
              total        used        free      shared  buff/cache   available
Mem:          852Mi       198Mi       372Mi        12Mi       281Mi       638Mi
Swap:         4.9Gi          0B       4.9Gi

however, if I use raspbian, also use free command, it show my correct ram size:

root@raspberrypi:/home/pi# free -h
              total        used        free      shared  buff/cache   available
Mem:          926Mi        46Mi       807Mi       6.0Mi        71Mi       823Mi
Swap:          99Mi          0B        99Mi

why ubuntu calculate my total ram size is wrong？how can I free up my "disappeared" ram size？

I love ubuntu ,I don't want to use Raspbian, please help.

Say thank you first.

Thanks.