A thing to consider about glass is that because of it partly random, partly ordered structured it is not necessarily electrically balanced on a atom to atom scale as would be the case with a fully crystallized material. This is especially true of a newly created surface. There are few things on earth quite as sharp as a freshly (freshly on the scale of microseconds) broken piece of glass. This is why, especially in situations such as an earthquake, it is a good idea to stay away from windows. The flying glass from a newly fractured window can fly right through skin and bone. Over time (seconds, minutes and for as long as a week) the electrically unbalanced nature of the new surface or of a scratch in the glass, begins to "heal" as the molecules at the newly created surface re-arrange their linkages to lower the surface energy. This healing also means that any glass particles created in a fracture or scribing operation can adhere themselves to another glass surface and become bonded there. As you would expect, every discontinuity in the glass surface has the potential to be a stress concentrator.
In general, glass strengthening processes such as thermal or chemical tempering are less effective on edges than they are on the body of a flat glass sheet. Part of this is geometry, part of this is how the edge was formed, and part is timing between the cutting and the strengthening operation. Though you would tend to think of glass as a static material, it does have its dynamic aspects. As I noted in a previous post, ion migration under the electron beam of a CRT eventually destroys the transparency of CRT glass to blue light. A bad cut, even for glass that is subsequently strengthened, even for glass that seems OK, will leave a weakened part.
The mobile device market periodically goes through periods of high display breakage. New suppliers or new staff have to attune themselves to the idea that their cutting process needs to be well engineered and maintained. Just because the glass is not shattering in their process or even in their customers does not mean that they are turning out quality for the end consumer.