6 things have swung it for me and I've booked both legs (as 2 single tickets)
1) over-excitement
2) the realisation the reward tickets are totally flexible, and for a very small fee you get all your avios and £££s back.
3) the return is on a 747 with NF, and I'm in 1A

4) the outbound is also on a 747, but with OF, so I'm trying out 1K

5) my understanding that West Coast USA reward tickets in J/F are stupidly difficult to find, and that these are on the
exact dates I needed
6) over-excitement