Because any company dumping huge loads of money on discrete Graphics Cards is going to make sure they have a market and a product to fit that market reasonably.
Any MP3 device, up to 120gb of storage ect ect..
Hard-drive, up to 2-3terabytes of storage.
MP3 is used for multiple productive decisions daily.
Hard-drive's have 1 use, store memory, and store some more memory.
On-board gpu, shares space and is designed inside of a multiple productive design USE for the user.
Discrete gpu, designed for ONE(1) use, to process any graphical output that needs to be displayed.
When someone design's either of those products they will think about different usage scenarios there for resulting in a overall difference in performance in different scenarios guarantied BASED ON PRIORITY'S.
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