Decathlon price difference

Recently, we went to the Decathlon outlet at Starling Mall to get a pair of swimming trunk for Ban. At first, the cheapest we saw was RM 59 but then I spotted RM 39, further down. He chose a few pairs to try. 

While waiting for him, I saw and overheard a dispute over price: a customer was using the self-check-out kiosk when he mentioned that price that he saw to the staff. Staff was surprised ("It's never that price", something like that) and went with him to presumably to check the price tag on the shelf, where I couldn't see. What I saw next was the staff emerging and threw away a small thing, which I surmised to be the incorrect price tag.

Once Ban chose a pair, we also used the kiosk to pair. FYI I'm a member of Decathlon i.e. I used their app. To our surprise, the scanned price was RM 59. Ban declined to sort it out with the staff and just paid for it.

Back home, I received an email from them, requesting feedback from me, in the style of a short survey. Where appropriate, I took the opportunity to state that there was a difference between the price tag on the shelf and the scanned price at check-out kiosk. I then consented to be contacted via email. 

Three days later, they emailed me asking me the quantum of the price difference, which I told them. Within the same there, they offered a RM 20 gift voucher which I can claim at the outlet by showing the email. I hope this voucher is either long-dated or has no expiry date.