One is actually recorded to glass and then stamped to blanks in a clean-room, and the other has a very high defect rate that can usually be corrected while reading:
DVD-Rs and BD-Rs are great, but they have some pretty significant limitations. They take forever to burn, they’re not that reliable (especially dual layer discs), and can’t support copy protection such as CSS or AACS, which are built into the DVD and Blu-ray specifications. While CSS and AACS can be broken, they do stop a lot of casual copying, and are required to be used, according to the terms of most license agreements.
Mass-produced discs are far, far more reliable than burned discs. They reflect more light, and they don’t have any of the flaws inherent in a recording that came from a cheap consumer laser and motor — both of which are subject to minor variations in AC voltage, dust, hard drive activity on the computer they’re attached to, and whatever else. The recordings made by a burner are always full of errors that must be corrected later, when the player reads them.
https://www.animenewsnetwork.com/answerman/2015-12-21/.96760
Pictured is a commercial pressing machine.
What? How? Wonder. 