It's not that Task Manager isn't accurate, it's just it reports the complete picture as one number, but the numbers it gives you for each program doesn't give you the whole picture if you add it up.
To understand the whole picture, it helps to know how memory is categorized in Windows:
- Working Set: This is the data an application has in physical memory. This is what counts towards the memory utilization %
- Private Working Set: This is the data in physical memory that's exclusive to the application. This is what's reported in the "Processes" tab per application and in the Users tab
- Commit Memory: This is the amount of memory the OS promised to give to the application if they need it. This is always slightly higher than Working Set. The total amount of commit memory that can be promised is the sum of how much physical RAM is installed and the size of the page file
Before deciding to add more RAM, it's useful to take an assessment of what's using it in the first place and seeing if you can take steps to reduce it.