Without a railcard, the trip from Bognor to East Croydon is £20-80 (Off Peak Day Single). To Victoria is only £13-40 (Super Off Peak Day Single).
There's no advantage in buying an early ticket for me. I can get something slightly cheaper to Victoria, but generally you can't break a journey early if it's an Advance ticket, so I'd have to go all the way into Victoria, turn around and cycle back to Croydon. If I buy a bog standard Super Off Peak Day Single to Victoria, I can get off early at East Croydon.
(Interestingly, as you say Adam, the Southern website is cheaper for advance tickets, even though it's clearly the same web-engine driving the site as the EastCoast website which I've used a lot in the past, in it's prevous guise as National Express East Coast).