You're right, and they probably never will; there may have been memory leaks that they do not want anybody to know about, and fixing these could in turn, fix "stability and performance." Memory leaks could lead to just about anything going wrong with a system, and thus getting a seemingly better update and getting all customers updated to the current version is a safe way from being targeted by black-hat hackers.
I usually assume vague 'improvements' (for most any software) as more efficient code compared to whatever it replaced, and thus details will never be revealed.
