MacBook Pro, OS X El Capitan - Photo Problems
The reason is that, when you sync photos with the app, pictures are copied in the Library, so it is actually created a new version of each photo. When you modify it in the app, you will only edit that copy, and not the original one. 

If you want to access the edited photos, you have 2 options:


Just drag the pic from Photos to a specific folder: the photo will be copied again, with all the previous edits you made.
You can directly access the Library. Just go to Finder > Pictures > Photo Library. Right click on it > Show Package Contents > Masters: here you have all the photos, classified by date. Now you can treat each photo as it were in a normal folder.