My hard drive capacity is 250GB, and it says I am using 247GB, but all of my folders add up to only about 100GB -- why?

Perhaps because you are missing all the system and hidden folders? Please provide a screen snap of whatever you are using to display the disk usage information.

To post screen shot do this:

1. Press COMMAND-SHIFT-4 which will change the cursor to crosshairs.
2. Hold down the mouse button and use the crosshairs to select the part of the screen you wish to capture.
3. Release the button and the image will be saved to your Desktop.
4. Click on the Camera icon in the toolbar of the forum message editor.
5. Drag the image onto the Choose File button and click on the Insert button.

Hard drive space and ‘Other’:

https://support.apple.com/en-gb/HT202867

Also, try to rebuild the Spotlight index: https://support.apple.com/en-us/HT201716