On the rare occasions that I have done this - er, just the once - only one way tickets were available. I bought:
- a ticket from Boundary Zone 6 to LGW (at Victoria)
- a ticket from LGW to Boundary Zone 6 (at LGW)
They were different prices.

I suspect that the Victoria person was better informed.
IIRC the actual boundary is between Coulson North and Coulsdon South.