A Question about wheel offset -- perhaps some of you can help me?

I've recently tried a 6.0x15in wheel with 20mm offset (Virgo Turbo), and 6.5x16in with 25mm offset (Hydra) on my '93 240. These are both OEM Volvo wheels, with their part nos. and sizes clearly stamped on them.

What confuses me is that the larger wheel has so much less clearance (to the inside) than the smaller one (it's so close to suspension pieces, weights on the inside edge wouldn't clear the pieces, as they're less than 1/4in away -- thus "taped" weights would be necessary). In contrast, the smaller wheel is between 1/2in and 3/4in away from those pieces.

So let me run some concepts through you folks to see where I'm misunderstanding the Offset concept.

If my understanding of wheel dimensions is correct:
1) The 6.5in wide wheel, compared to the 6in wheel and if they had equal offset, would only extend 1/4 inch further inside than the smaller one because the 1/2in greater total width is shared among both inner and outer sides.
2) But since the larger wheel also has a 5mm greater offset, that shifts it outward by 5mm (which is approxmately 1/4 inch).
3) Thus, I would expect that both wheels should have their inner edge close to the same plane (and their outer edges on very different planes, 1/2in apart, from their difference in width).
But apparently, they don't.

So what's wrong with my logic, or with my understanding?

Thanks for helping.