camera roll has 900+ pics but it shows 1600+ in iTunes. Why? Please help.

I'm using Iphone 5 and latest iOS.

My camera roll shows 900+ of photos but in iTunes it shows 1600+ of photos with amount of 3+GB.

May I know what's happening? How could 900+ pics can get 3+GB and it shows different amount of pics in the iTunes?

I have deleted some of the photos using 3rd party software recently from 1600+ of pics to 900+ but the iTunes still shows 1600+ of pics with an amount of 3+GB?

What can I do about it?