There is one way this weird array of Macs might make sense... If chip supplies were constrained (but the M3 Pro and M3 Max were no MORE constrained than the base model).
Mac sales volumes (in units, not dollars) go something like (this is an educated guess):
MacBook Airs (by far the best seller)
13" MBP
Mac Mini
14 and 16" MBPs
iMac
Mac Studio
Mac Pro.
If the iMac is a REALLY terrible seller, it might sell less than the Studio (which has been successful for an expensive pro desktop) - but I don't think so - it's much cheaper and aimed at a bigger market.
I also don't think the big MBPs outsell the Mini (in volume terms - they probably do pull in more revenue), but that's the other one I'm not quite sure about - laptops usually outsell desktops, but the 16" MBP has an average selling price close to four times the Mini's.
If you had no chip supply constraints, the obvious upgrade (which gets all the best sellers before the holidays) would be Airs/13" MBP/iMac (possibly ignoring the Mini because it needs the M3 Pro chip as well as the base M3). The 14" and 16" Pros are relatively new and need different chips, and the Studio and Pro are brand new. This also keeps the upgrade cadence of base first, then Pro/Max, then Ultra).
Throw a fly in the ointment - chip supply is severely constrained, but the Pro and Max are ready alongside the base chip. If they upgrade the Airs, delivery dates will immediately shoot to March or something, because they simply don't have the chips.
Suddenly, this weird group makes sense... The iMac is REALLY old (I don't know why it missed the M2 generation entirely), and updating it gets at a sore spot in the lineup. Maybe they give it an M3 Pro option at the upper end, maybe not. They have chips (actually, chip SLOTS at TSMC that they can use for the chip of their choice) for one more mid-selling model. They can choose the Mini, the 14" and 16" MBPs or MAYBE the 13" MBP (it may sell too many units).
Now it makes sense to update the 14" and 16" - they bring in a lot more $ per machine than the Mini does. They are also favorites among a highly visible group of creative users.
The other possibility, which has been alluded to, is that there are two versions of the 3nm process, and the second one (not ready yet) has better yields/higher volumes. Could that second process also be the only one that runs cool enough for the Air? The Air (and the iPads) are extremely thermally constrained (not to mention cost constrained). What if there are actually TWO versions of the base M3 - one that would work in anything, and one that won't work in the Air due to thermals? The Air HAS to wait for the high yield/good thermals chip, but the iMac doesn't. The Pro and Max are based off the first process anyway.