It seems to me that it is completely random in nature. The main reason I heard from tech support was that the cards were of different denominations. That is, I

It seems to me that it is completely random in nature. The main reason I heard from tech support was that the cards were of different denominations. That is, I took only cards of 100 dollars would be all right. I just talked to a colleague, about which I wrote, she used the same denomination cards to buy a phone (she did not do it on purpose, but it just happened that all the cards were $ 100)