The £1.80 for a peak Z23 ‘Tube’ journey seems correct to me: as Mike says, “most of London Overground is treated as if it were the Underground”.
Balham has both Tube and Rail gates so the fare is different depending on whether you use Balham LU or Balham NR or both.
From Balham you can get the £1.80 fare if you touch in at Balham Tube (not Rail), get off at Clapham North, walk to Clapham High Street Overground, then continue to Clapham Junction and Shepherds Bush NR, so you’re not using actual NR at all, only Overground.
From Balham NR, changing at Clapham Junction then onto Shepherds Bush NR, you are using actual NR hence the higher cost.
From Tooting Broadway, there are only Tube gates, so it’s £1.80 for Tooting Broadway – Clapham North – Clapham High Street Overground – Clapham Junction – Shepherds Bush NR (no actual NR).
For Tooting Broadway – Balham LU – Balham NR – Clapham Junction – Shepherds Bush NR, it’s £2.90 peak because it involves actual NR.
Or am I not understanding the situation correctly?