@prn the issue with some of the "rulings" (which usually are not rulings at all, but rather are just examples) is that they can mislead people in some cases, since they don't explain why something is as it is.
I think a key to your understanding is to check out the comprehensive rules section that details the actual steps to "cast a spell." (601.2a-i)
When we consider that Trinisphere's effect will take place in 601.2f, that is, the step in which we are determining the spell's total cost, but Delve and Convoke apply in step 601.2g, because they are not "alternative costs" they are an alternative way to pay mana for a spell with that ability.
So, Delve and Convoke function in step 601.2g which, being after Trinisphere's effect is checked in step 601.2f, the key final sentence of that rule comes into play, "Then the resulting total cost becomes “locked in.” If effects would change the total cost after this time, they have no effect." There is no recursive check, once in step g, we never go back.
Is it intuitive? Not really, but it's just how it is.
EDIT: Here is a link to the current and correct rules document (third party ones are often outdated and wrong): http://media.wizards.com/2017/downloads/MagicCompRules_20170605.txt