When you include "hidden files," Windows continues to exclude a subset of hidden files called "protected operating system files," unless this default behavior is also explicitly disabled.
Including hidden files in your total will definitely reduce the discrepancy you currently see between the two storage-usage totals.
There are some potentially massive files in the root of your C drive that aren't being included right now:
- pagefile.sys
- swapfile.sys
- hiberfil.sys

However, even with both hidden and protected operating system files included in the total, the sum of the individual files and folders may still fall short of the total amount of space reported by Windows for the entire C drive.
If this happens and you still want to find out exactly where your missing gigabytes went, install TreeSize by JAM Software. This free utility indexes your files and folders in seconds and will show you how Windows reaches that 155 GB total.