Why is Photo Stream content different on each of my devices?
As the Apple website reports, iCloud stores your pictures in Photo Stream for 30 days.


  iCloud manages your Photo Stream efficiently so you don’t run out of
  storage space on your iPhone, iPad, or iPod touch. If you have Photo
  Stream enabled on your iOS device, every single photo you take appears
  in a special Photo Stream album that holds your last 1000 photos. You
  can delete any photos you don’t want from the Photo Stream (Photo Stream photos downloaded to an iOS device with iOS 5.0.1 or earlier must be deleted manually after updating to iOS 5.1 or later.). To touch
  up a photo or keep a favorite shot permanently, simply save it to your
  Camera Roll. iCloud stores new photos for 30 days, so you have plenty
  of time to connect your iOS device to Wi-Fi and make sure you always
  have your most recent shots handy.


After 30 days pictures are removed from iCloud but not from the Photo Stream album on your devices.

If you remove a picture stored in the Photo Stream album from one of your devices


Before 30 days, the picture will be removed from all your devices
After 30 days, the picture will be removed only from  that device, because the picture is not anymore on iCloud.


You should also consider that pictures are removed from your Photo Stream albums on your devices when they reach the 1000th picture.


  iCloud pushes all your photos to the My Photo Stream album on your
  devices and computers, and manages them efficiently, so you don’t run
  out of storage space.
  
  Your iOS devices keep a rolling collection of your last 1000 photos in
  the My Photo Stream album. From there, you can browse your recent
  photos or move the ones you like to your Camera Roll or another album
  to keep them on your device forever.
  
  Since your Mac and PC have more storage than your iOS devices, you can
  choose to have all of your My Photo Stream photos automatically
  downloaded. In iPhoto or Aperture preferences on your Mac, be sure to
  select Photo Stream > My Photo Stream > Automatic Import. All of your
  photo stream photos will be imported into your Events, Projects,
  Photos, Faces, and Places folders in iPhoto or Aperture. On your PC
  with Photo Stream enabled in the Control Panel, all of your photos
  will be imported into C:\Users\\Pictures\Photo Stream\My
  Photo Stream.


N.B.1 You can always reset you Photo Straem from www.icloud.com.

N.B.2 You should satisfy the following requirements


  Deleting individual photos from Photo Stream is supported by iOS
  5.1 or later, iPhoto 9.2.2 or later, Aperture 3.2.3 or later, and Apple TV Software Update 5.0 or later. You will need to manually
  delete photos you downloaded to iOS devices using iOS 5.0.1 or earlier
  after updating to iOS 5.1.