Misleading Advertising - sold more than available stock

I ordered 2 items from Makro on 22 November 2017 and received my order confirmation after paying. Later that day I tried to order the same item but was told that they only had 51 units and that it has been sold out. On 29 November 2017, I received a general email about their delay which I understood as it had been a busy period. I received similar emails on 6 and 8 December from them apologising for the delay and I continued waiting. On 19 December 2017, I called them about my order and they gave me the option of waiting for stock or taking a refund. I opted for the former. They emailed me today to inform me that they do not have any stock and offered a refund. If Makro only had 51 units available and I ordered a unit, why have I not received it? Was it sold to someone else? I've seen the item in stock at 2 different retailers so why is Makro unable to source the item?