Woolworths 55 days interest free account not 55 days interest free

I opened my store card in June 2019, mainly because their website advertises this as a 55-day interest free account. My first purchase was made on 6 June 2019. However, on my statement for 10 July 2019 I see that interest has been charged. Upon following up on this matter, Now bear in mind that 55 days from my purchase is only 31 July 2019. I called the call centre to get this charge reversed. I was informed by Loraine Buys (manager at the call centre) that I need to pay my account in full, every month, in order to not have interest charged. Now I would like to understand where the 55 days interest free come into play. I think this is false advertising if one needs to have your full account settled at the end of each month to avoid interest - no month has 55 days therefore the 55 days interest free is indeed false.