Deep groove ball bearing can be affected by:
Material
This is not a simple "GCr15" that can distribute everything. Steel quality varies from mill to mill.
craft
Even though a factory may have a different process, just doing it slowly or faster will make a difference.
design
Drawings vary, and when it comes time to try to save costs, there are more derivatives
equipment
Includes repairs including heat treatment equipment, turning, grinding, superfinishing, cleaning, automated assembly
worker experience
Chinese workers are not as rich as they were a few years ago and wages have risen a lot, but al all factories still lack experienced workers
Quality testing equipment
different standard parts,
manage
It's an attitude towards quality control, not simply an ISO certificate that can be easily "purchased"
Part quality
This is very important for insert bearing's bearings, balls, rollers, cages, seals, greases, and even anti-rust oils.