This surprises me a bit. I haven't burned neatrly as much stuff as you have, but I've occasionally retested a lot of discs, e.g. after firmware change or with new KProbe or CD-Speed versions. And I haven't really observed such effects with my "disreputable" LiteOn SOHW-1213S (@1653S).
Now you've used 2 different tools for the test. I know that KProbe and CD-Speed show essentially the same results on LiteOn drives (except for "known idiosyncracies" with averages). DIP, on the other hand, had problems with LiteOn drives in at least one version. I'd like to see a comparison with the same version of DIP for some of these discs (e.g. the first one, where the curves are similar, but the DIP sums are only half as big), just to eliminate this as a factor in the tests (once burned, twice shy

Could you say a couple of words as to how you've stored these discs? Jewel case? cake box? on the floor? I'm sure the humidity in Vancouver is nothing like Singapore (and the temperature isn't either).
Thanks for the effort!
G