I'm thinking of buying a Gravity Avenue C, but I'm puzzled by the weird frame structure in the back. Why don't the two triangles match up (highlighted in yellow)?

you're asking for a justification of that? I dunno, maybe they wanted to ensure there were warranty returns.

I don't think the mismatch is structurally significant. My guess is that the rear triangle was tooled with certain angles and is used on different size frames. On a taller frame it will hit the seat tube a little short of the top tube. Or.... the designer just designed it that way. If that was such a big deal, step-through frames would fail all the time, but they don't.

I wouldn't worry about it.

it's not that uncommon among production bikes. Since there is no reinforcement of the seat tube/top tube junction, the seat tube is probably heavy enough to take the load at the seat stays without crumpling. If it was a lightweight steel frame, I would question the builder's judgement.

The bottom line is that it's just a marketing hook. It's designed to catch your eye. The maker may or may not try to justify it as "better". In reality it's a gimmick and most likely harmless.

I agree with the posts so far, though that kind of denial of the importance of details applies to a lot of custom frames also. What is the role of the Fleur du Lis on the lugs kinda stuff. Or the lugs.

In general that kind of thing looks intended to soften the rear end, and harden the front end. Why that would appeal in this case could just come down to marketing hooks as mentioned. Or it can be that for some reason they tried it, and for whatever reson that particular bike worked great. Like going into a shop and riding three seemingly identical bikes, and yet one feels alive and the rest are like riding a section of guard rail.

Why a deeply ovalized and curved tube. Sorta nets out at higher weight.