Synchronize Contacts on iPhone and iPad
Each device has a local store of contacts. To synchronize things on each device - you can turn off Exchange sync and then see all the contacts that remain are local.

When you re-enable exchange sync - copy them one by one or choose to sync them all up to the cloud. Then repeat for each other device.

By setting the default place to store new contacts - you can avoid storing contacts locally or you can set time to clean up periodically if you don't remember to choose where to store each contact as you add them to the device.