Yes, you describe it correctly. On half the levels it shifts it up by one. Considering the whole post what that half the time it shifts it down by one, I hope you can see how that's a big deal.
A Paladin 1 / Sorcerer 1 has the same spell slots as a Sorcerer 2 using your math, plus all the extra goodies a half-caster gets because they shouldn't be advancing their magic as quickly.
Except they don't. A Paladin 3/Sorcerer 1 doesn't have Paladin 4 features, nor does it have the same features as a Sorcerer 3, let alone a Sorcerer 4.