Iphone reads large amounts of 'other' in Itunes. I have to restore to NEW every other month to fix it! WHY?

Iphone reads large amounts of 'other' in Itunes. I have to restore to NEW every other month to fix it! WHY? I have went over this over and over again with Apple. Actually, one time, they took over my computer to check it out and couldn't figure it out so they just mailed me a completely new phone. The other may read 7.5 GB and then, when I restore to BACK UP it reads an enormous amount like 11.1 GB! It only gets corrected when I restore to NEW and that screws up all my passwords, all my apps are askew, yadda yadda. Any suggestions?