Is this still the case? Windows used to have an almost 45° lighting angle, but more recently (MacOS, Material Design) the more common case is the light source shining from the top.
Top-left was used in the past because the only lighting hints was the edge colours, and this looks weird with a light that is directly above, so typically a 45-degree light was assumed.
In modern design, there is very little edge colouring used, and instead the light determines only shadow positioning. And shadows look perfectly fine with a light positioned directly above. This placement feels a bit more natural, as it doesn't introduce an arbitrary sideways bias.
The use of bevels has similar attributes to the edge-highlights of win 95, where putting the light from the top can make the left/right edges indistinct.