For season tickets the price for u-18s was 50% of the adult rate, u-10s was 25%. In fact it still might be - I've not checked. I'm pretty sure there were match day tickets sold at those age brackets too, although I might be wrong.
And maybe it would all balance out if all tickets were sold at different age groups. We don't have access to the data that would confirm that though...
If that were the only scenario in play then you'd be correct in highlighting it as wrong.
However, if an adult season ticket is put into the exchange, the ST holder will receive c£30 back (depending on the ticket and the match category) whilst the ticket may sell to an u-18 for £15 (depending...