Storage not the same on computer for iPhone?

So I have the iPhone 5c (yes it's old) and I'm always getting notifications that my storage is full. If I go to my settings it shows me that I only have a few MB's left. However, whenever I plug my phone into the computer and open iTunes, it says that I still have entire GB's of space open. These can't both be right, so I'm not sure what the issue is.

IPhones build up a lot of useless cache over time, this is data that makes apps load up faster by storing certain data such as photos. Once your storage starts getting full, it should start deleting cache files however it doesn't always do this.

The best way to fix this is to create a backup and then restore from that backup. Cache files don't get backed up so they will be deleted and everything else will stay

They storage is being expressed in 2 different ways. One is what you have ACTUALLY saved. The other is what has been saved not only by you, but the phone's operating system, back-up frags, etc. Get Dr. Cleaner app by Trend Micro (free!). It will remove those bits and pieces and free up more space for you - until you get a newer phone with MORE storage.