The real memory of iOS app will automatically reduce?

I used activity monitor to check the real mem of my app. when the app launched, the real mem reached about 100 mb. but after some time , the device went to sleep. Then I actived the device and reopened the app again, the real mem was only about 30 mb. so strange!

I know the real mem including both clean and dirty memory, the clean memory can be freed by system! but first, there isn't about 70 clean memory, and also the clean memory will be returned the App reopened.

so my question is If the real mem stat in activity monitor can be trusted?