I think what Apple means with "no known issues" is what you say, that the problems that they set out to fix in a point release are fixed. It's very unlikely that Apple or anyone else can ever claim to have fixed every possible issue a piece of software is likely to have. All they can do is pick a set of issues that are the most severe, effect the highest number of people, is consistent with the direction of current and future OS, and can be fixed in a given time frame. Then they just fix those issues, check with others to make sure that the issues are fixed, and check with others to make sure it doesn't break something else. Their bar is probably as long as it doesn't make things worse then it passes, it doesn't necessarily make things better for everyone or everything.
I believe after Tiger hit double digit point releases 10.4.xx and the counter overflowed, Apple changed the version counter in Leopard so that double digit point releases can be detected properly. If they are going to stop after 3 more point releases, I guess that work wasn't needed afterall.
If Apple stops point releases for Leopard after Snow Leopard as they've done in the past, I guess that'll mean they are going to stop security updates for Tiger soon after. After Leopard was released, Tiger received 1 more point release and Panther 1 more security update. I was hoping that with the increased install base of Macs now, Apple would continue supporting security updates for Tiger. I don't believe Jobs has shown OS breakdowns recently and I'm pretty sure the last one shown, Tiger was still in the majority, even though Leopard adoption was significant. Admittedly though, for Apple to remain nimble, they really should keep forcing people to keep to the most recent 2 operating systems.