Both S4 and S5 are updated to Lollipop, you may think why aren't either of them on 5.1.1? You may even think why is S5 on 5.0 and S4 on 5.0.1
That is something I can't explain that well, but I'll try.
I don't know why Samsung develops their updates like that, but they just do. They choose a random Android version and they develop and update for their device, but it won't be the latest Android version. However, for example, the S5 only has 5.0 builds available right now.
On SamMobile, if you search for G900F firmwares, a list will come up showing Android version and build dates. There are firmwares from last year with 5.0 and firmwares from a few days ago with 5.0
This means, Samsung includes any fixes, performance improvements and security updates from 5.1.1 and puts them into the same 5.0 build and pushes an update.
Your second point, why doesn't the UI on S5 look like S6?
Same reason as Apple.
However in this case, the S5 can easily handle the animations and UI elements of the S6. But Samsung still refuses to make all their phones look the same.
In Apple's case, the iPhone 4S on iOS 9 and the iPhone 6S on iOS 9.
Its horrible and Apple decided to not include some of the features in the 4S, because its tiny 512MB RAM couldn't handle it.
I read somewhere that the:
Galaxy Note 3
Galaxy S5
Galaxy Note 4
Galaxy Note Edge
Galaxy Alpha
and some others
Will get Marshmallow in Q2 of 2016.
Galaxy S6
Galaxy S6 Edge
Galaxy S6 Edge+
Galaxy Note 5
and other newer gen devices
Will get Marshmallow in Q1 2015 and its possible we may see signs of it around December.
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