Why do updates depends on carriers / manufacturers?

[Blade800]

Sorry guys dont know where to put this question. I was talking to a friend and he was telling me that this is BS and theres no chance how carrier can influence update (for ex. 8.1 update). And I didnt know how or why I just know that it is true. Can you explain me why is that ?

 

[agd87]

I think they have to test it and make sure it works well and then they have to apply their own firmware to the update. Their firmware will add custom settings, images, splash screens things like that.

 

[Blade800]

Can someone elaborate more pls ?