Why is my other storage on my iphone displayed as so high on itunes after a back up yet not shown on my actual phone?

Hi i just backed up my iphone on itunes for the first time. It had been backing up automatically via icloud but this time i selected to back up to computer.Once this had finished i was surprised to see that my 'other' storage was displayed on itunes as 27.8 GB leaving only 8.71 GB free. I checked on my actual phones storage and this works out fine with the correct available storage.

Is there any reason that itunes is displaying this when my phone is not and how can i fix it as, although i have not tried to, i would presume that if i tried to transfer say 9GB of music to my phone itunes would not let me even though i should have that space available.

Any support would be appreciated

Thank you.