The beveled case head on what looks like an 8mm Lebel case in the image is known as the Mauser A Base case head. As far as I know the beveled head originated from the 11mm Mauser black-powder cartridge and was not uncommon on a few other black-powder cartridge cases and those that transitioned to smokeless powder such as the 8mm Lebel. The Lebel case is tapered with curved case walls similar to what the image shows although camera angle maybe playing tricks here.

I don't know why the beveled head was used, perhaps came about because some very early metallic cartridge cases were two piece with a beveled rim swaged to the case cylinder to form the head and pocket for the primer.