Hi,
Oyster always charges up to the cap of the highest zone used. This sometimes can cause a perceived overcharge (when you come in from zone 6 and don’t return beyond zone 2 for instance), but in this case you’re still better off. The single zone fare with a railcard is £1.15 off-peak and £1.90 peak, so if you want to use the zone 1-2 cap you would have paid £5.60 + £1.90 + £1.15 = £8.65. Instead you’ve been capped at £6.60.