Not all temporary files can be deleted. Some of them are system swap files. The clear button should remove any temp files that can be removed.
On my 920, it ranges from around 500mb to 900mb. Goes up and down. Not sure why and how. I believe the system temporarily allocates unused space to speed up processes and gives it back as required.
Not sure whether you are using the 'Storage Check' application under 'Settings' or the one available thru' the Beta Labs. If its the former, try THIS . To download you need to log-in using the same credentials that you use on this Forum. Once installed, the application will be available under the 'Applications' list (Left swipe n Home screen) ..
Also have a look HERE
Thanks for your tips. I've tried beta version but with same result.. And that FAQ article seems to be created before Storage Check was implemented into Settings.
Perhaps you could monitor it for a while and if it continues to increase and never decrease then you could consider resetting which should bring you back to square one but does require a bit of work.
Settings>About>Reset your phone. This will work but will also wipe your phone. Easiest is to backup essential data only to the cloud before resetting for restoring afterwards.