Thanks, iOS is notorious for caching such that if you add up all your apps under usage it never reconciles with the claimed 'Amount Used'. I don't think all cached data ever gets revealed or fully released. My available space can drop by a hundred megabytes out of nowhere for no obvious reason - although I strongly suspect iCloud backups and app updates (most often reason for corrupted data is failed syncs but I never sync with iTunes as I use match). Mine doesn't reconcile by 2 to 3 gigabytes (not as bad as Ggator at 9.4!). It's a trend I have noticed across many iOS devices & versions that I/family members have owned or continue to own. Anyway that is slightly off topic.
I am due to get my watch tomorrow, am very much looking forward to receiving it. But I do remain concerned the apple watch app could have a potential to take up alot of storage space on your actual phone probably through caching as it communicates with the watch. It remains to be seen once I start using it. I noticed currently apple watch app does not appear under usage on the iPhone. Maybe it will once a watch is paired and I wonder if it will be a true reflection of how much storage is being used. However, Meg - that does sound promising that you haven't observed a noticeable amount of space usage so that gives cause for optimism.