I stopped expecting consistent, deterministic information about storage usage from Apple somewhere around the release of High Sierra and APFS. Howard Oakley had an article about the challenges of measuring available storage on Macs fairly recently over at https://eclecticlight.co/, though I can’t find the link at the moment. Nowadays, I just make sure that I have qualitatively “ample” storage space on all of my devices and on iCloud. Defining “ample” can be challenging. On modern Macs, I try to keep a minimum of 10% of the nominal capacity free and to avoid going below 50 GB free even on systems with 256 GB SSDs.
This is not about storage on your iPhone. It’s about how much your iPhone backs up to iCloud. That should be known down to the MB. Apple displays that information in at least 3 different locations and not a single one of them is in agreement.