What contributes to my iPhone iCloud backup size?
Per Apple, this is what is backed up:


Purchased music, movies, TV shows, apps, and books
Photos and videos in your Camera Roll
Device settings
App data
Home screen and app organization
iMessage, text (SMS), and MMS messages
Ringtones
Visual Voicemail


Things like purchased content, Messages.app, ringtones, and voicemails are not actually explicitly listed in the backup screens for you to see how much space will be used. This would account for the difference.