Apple's new APFS file system, and Apple's user-facing representation, is wildly inaccurate and confusing.
It leaves us here in the forum trying to explain complicated technical details that no one understands.
You probably have made copies of one or more apps. Maybe you've made a duplicate of your entire applications folder. That would explain why your applications occupy more storage than you have.
To paraphrase the Great Steve Jobs, you're looking at it wrong. Or rather, Apple is showing it to you incorrectly. Apple is simply taking a list of your apps and adding up the size of each. But because one or more are duplicates, they don't actually take up any additional space on your hard drive. The end result is that 794.72 is less than 500.
That being said, one of those values actually is correct. You do only have 17.71 GB of free storage. That's critically, dangerously low. You need to delete and/or archive some files before you suffer unrecoverable data loss. If you didn't already have a Time Machine backup. This is when you start.
And I'm not done. 😄 As you delete files, you'll notice that your free storage actually goes down. Don't worry about that, keep deleting. You won't see the result in actual free storage for a couple of days. What am I saying? You won't ever see the result in actual free storage because Apple only tells you about its made-up fantasy called "available" storage. But once you get to 100-200 GB free, then your available store will be more accurate and reasonable.
Just keep your eye on that top graph for "Macintosh HD". That's the part that's correct - somewhat. Ignore the graph, look at the two values. You've used 476.67 GB of 494.38 GB on your hard drive. Once that first number drops below 300 GB, you'll be ready to run!