Multi-classing doesn’t generalize Single-class spellcasting?

I am aware of our go to question about spellcasting and multiclassing, and I am aware of the rules. I am nonetheless confused about something:

If we use the multi-class rules, in particular

You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, half your levels (rounded down) in the paladin and ranger classes […]

So, a 5th level paladin contributes as a 2nd level caster (5/2 = 2.5, rounded down 2). A 2nd level caster would have 3 first level spell slots, and that’s all. However, a 5th level Paladin has a spell slot table that clearly reflects a 3rd level spellcaster. In general, it looks like the Paladin level is being rounded up when defining the class’ spellcasting table. The same can be said to the Ranger and to the "1/3 casters" (Arcane Trickster and Fighter).

So, when you multiclass, the half-casters and 1/3 casters contribute less to the spellcasting levels than their own progression suggests. This means, for example, a 3rd level Paladin and 2nd level Ranger, although "jointly" count as a 5th level "half-caster" (you sum them before dividing and rounding), when multi-classing the character is only a 2nd level multi-class caster and doesn’t have 2nd level spell slots, even though a single-class 5th level ranger or a 5th level paladin would both have 2nd level spell slots.

This is weird to me, so, to start with, is the math right? Am I missing something? Or multiclassing is, in fact, inherently harming the spell progression provided by the not-full-spellcasters, in a way that 5 levels of different half-casters is simply worse than 5 levels of the same half-caster?