So I, uh… decided to run over your numbers assuming a Hoard Quality of 1, and then assuming ingot drop break points WERE crossed (so a lower lowball estimate of 6 potential Legendary/Mythic ingots rather than 8 – that’s what you meant, right?).
For Quality, I doubled the percentages attributed to Glory, Souls and Ingots, and quadrupled the percentage for Gold (Quality 10 gives x2 and x4 to these, respectively; dividing the estimate by 2 and 4 multiplies the percentage by the same).
For Ingots, I obviously just recalculated assuming 6 instead of 8 and replaced those percentages, doubling for Hoard Quality.
That came out to the mailed rewards being 94.3% of the lowball estimate, and then ~100.6%, after the ingots, not taking into account the +0.25 to multiplier.
While fishing-for-numbers-that-add-up-to-100%-and-then-deciding-that’s-the-correct-answer is methodologically flawed, the main ways I can see it being drastically misguided would be if the possible rewards were valued differently, and were compensated disproportionately to their normal drop rates – e.g. if, hypothetically, Glory was considered to be the most valuable, and normally only drops 10% of the time, but we were given 20% as Glory. So, possible.
IF these are anywhere close to the assumptions made for mailing out compensation, it would make
look a little bit like
I.e. Not technically wrong, just needing a little more context 
.
Realistically, it seems more like, “the least generous we could possibly be, while still drawing from legitimate potential values [no full clears, lowest Hoard Quality] so as not to overcompensate newbies[/anyone?] or upset our publishers”. 
Having said all of that…
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Losing out on ~25+% of what would have been earned from the Boss Room (not taking into account extra time/Gems to finish the event, though) isn’t thaaat much in the scale of “total rewards earned from the event”, given that x0.25 (boss multiplier) is 6.25% of x4 (non-lowball multiplier estimate), and 1 Chest Level is probably ~1/6-7 of the average expected Chest from the Faction (i.e., ~4% of total Chest rewards) (I take it back, that adds up).
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An easy way to ensure varying Hoard Quality didn’t affect compensation would be to give it all in the form of Chaos Shards – a solution that may(?) have left many Delve regulars unhappy (as stated earlier in the thread – although I’m guessing Quality 1 rewards weren’t what they had in mind) (depending on the resultant calculations, as well).
Nonetheless, yeah, below the lowball estimate isn’t exactly what comes to mind when “as generous as possible” is said.
Disclosure: I also didn’t participate in the event, beyond level 20-30.
Wording seems inverted? 
E.g. “What was mailed out adds up to only 77.3% of these [tangible values of what could have been lost at the lowball 3.5 multiplier estimate]”