from wageningen to amsterdam, what's the "reduction" price

sorry, i only can type english.

next Wednesday i need to travel to wageningen, later on, i plan to go to amsterdam.
I checked the ns.nl. one thing i don't understand is:
Train fares   2nd class
full fare     21,20                        reduction   12,70   
e.g. i want to take the 2nd class for the return ticket, what's the requirement for getting reduction price?
they didn't say it on their webpage.

Does anybody know about it?
Thank you in advance!

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周1-5,早上9点以后可以用打折票~ 周末不限

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