Regarding the way that the lower part of the extractor is ground away, here are a couple of thoughts.

First, is that these rifles use a staggered cartridge positioning in the magazine, meaning that the cartridges do not slide straight up the center of the bolt face as they are being fed into the chamber.
Each cartridge will come up out of the mag off center and at an angle due to their staggered magazine position.

Second, is that while the '98 style bolt diameter remains the same for different chamberings, the diameter of the cartridges and the cartridge heads do change.
The different diameter cartridges can lead to different width magazine boxes.
This can effect how far off center the cartridge is when it needs to slide up the bolt face and under the extractor claw during controlled feeding.

Third, is that CZ's are mass produced and would require the easiest method to obtain some degree of reliability with feeding as the come from the factory.
I think this could be the reason for that relief at the bottom of the extractor claw.
It could allow the cartridges to feed better from the right side of the magazine, allowing the rim to more easily find its way under the extractor claw as the cartridge is released from the magazine.

All just opinion of course.