The upgrade path is annoying, for sure, but there might be internal reasons for the paths they offer. Check out my post on ABS for starters.
Whenever you get a card, Capital One looks at your credit score and their own internal calculations to decide if you're worthy of packaging into a portfolio to offer to investors. They have many "tiers" of risk/return to offer. They were at the forefront of this madness a long time ago while other conservative banks who use their own money (mostly) would just deny folks without great credit.
The issue here is that once your card is in a 'tranch', it's hard to get out of it -- it's very common for someone who goes from putrid scores to prime scores to be stuck with a putrid card limit because their new fancy scores are helping sell the putrid investment package.
At some point, someone closes a card so there's room in that tranch to move someone else up, so a PC may in fact allow Capital One to take someone out of one group and sell them to another. But it's feasible that an individual borrower may be in two totally different tranches with two cards because of a significant change in their credit score/profile between apps.
So maybe with the Platinum you were in a subprime group but maybe when you got QS you moved to a better one -- and it's possible that with Platinum the card is backed up by investor money while your QS was backed by Capital One's money. So instead of letting you go Platinum -> Venture directly, they wanted to keep your Platinum with the investors (as a QS) but keep your QS with their own money (as a Venture).
No way to tell, you can't demand disclosure on this stuff unfortunately. I've tried writing letters requesting what tranch investment package I am in and no response ever, lol.
