Most of your questions should be answered here:
http://arcengames.com/mediawiki/index.php?title=AI_War_-_Strong_Vs_WeakAs far as the "Why do some ship seem to be less efficient towards another ship type at a higher level of Tech?" question, that is something that can cut both ways. Different ships progress differently through the mark levels. So, in other words, some might get cheaper but not progress in strength as much, others might become better against groups but not appreciably better against individuals, others might become much stronger and much more expensive (expense not being considered by strong/weak data, though).
So when you factor all those different approaches in, you see the numbers differ a bit between the mark levels. However, it's generally not something that would affect your strategic decisions even, just something that is based on these numbers being the result of a complex simulation with differing inputs at each mark level. Not something to worry about overmuch -- if anti-armors are good against bombers at mark I, they still will be at mark IV, and the slight differences in the raw strong/weak numbers would not be something that should really make you treat the mark IVs different from the mark Is, for instance.
Hope that helps!