This is a strange anomaly. If you look at a fare finder and go from Liverpool Sreet (NR) to Hertford East for 16-18 Zip Cards, their cost is £4.80 (peak) while the 11-15 Zip Cards is 10p more at £4.90 (peak).
Also, going from Ware to Hertford East (off-peak), 16-18 Zip is £0.90 and 11-15 Zip is 10p more at £1.00.
Why are 11-15 Zip prices 10p more expensive than 16-18 Zip.
Why does this exist?