Based on diminishing returns, adding the extra stats from a defense on-use relic, you go from 30 to 32% defense.
Something must be missing from your calculations, because that just does not match with what I can see.
I go from from 27.95% to 34.27% when activating DG relic of imperiling serenity.
Now if someone has more defense rating without relic activated they should have also more with it.
There is no way you can have less percentage with more rating, if the base percentages are the same. It just does not work that way. Further increases at higher percentages get gradually smaller, but they are still increases.
So if at 27.95% I get +6.32% from it to push it up to 34.27%, then someone with 30% should get, I would estimate, something still over +5% pushing them to over 35%.
30% defense chance also is not any kind of a cap. There is of course a theoretical cap of 30% gained from the rating alone
, (which would mean 46% actual defense chance) but since that is completely unreachable, it is largely irrelevant.
30% of the actual defense chance is not any kind of point of significance.
A properly specced assassin tank with 0 defense rating gear does have 16% defense chance (10% base defense + another 6% from various points spent in the skill tree) . That means that at 30% defense chance, a properly specced assassin tank does have only 14% defense chance gained from the defense rating stat.
14% is not even close to being halfway up to diminishing returns curve, and one technically could go (not saying that they should) much higher without DR getting very high.
Maybe without those 6% it might be a bit different story.