Sorry that Photoshop is displaying less available memory even though it is allocating 100% of it.
Which exact version of Photoshop are you using? Latest is 22.4.1
I have couple of questions to better diagnose the issue:
How much free space do you have on your primary harddrive?
The RAM usage and available memory could be reported wrong by system info, could you please post a screenshot of Task manager in Windows showing the memory used by Photoshop while it is editing/processing an image?
Akash, troubleshooting will not resolve this one. There are a few of us noticed the same in v22.4.x. mine reports 11GB less total RAM in v22 than in v21. I haven't raised a bug report as with 128 GB RAM installed I will still not run short.