Why bellows breath repeat pays deflect once but falling star twice?

Partly — the "once" side of that comparison is out of date. Bellows Breath with Repeat should also charge Deflect twice.

Deflect is charged per time your spell chooses the unit, not per spell (Rule 809.1.c), and Rule 820.2 makes it explicit that the second Bellows Breath execution gets fresh choices made during finalization. Choosing the same Deflect unit again on the repeat is a second choose, so it's a second payment.

The apparent asymmetry is really about when the choices happen. Falling Star has two separate "Deal 3 to a unit" instructions inside a single execution, so it chooses twice with no Repeat at all — that's 2 payments on one card, and 4 if you also repeat it. Bellows Breath is one instruction that chooses up to three units once each, so its base execution costs 1 payment per unit and Repeat adds exactly one more payment per unit. FAQ #12878 confirms the per-choosing reading and explicitly flags the older "only once per spell" entries as conflicting with current rule text.