Queen Cell opened at the end tip and the side - Explained

I believed that all Queen Cells opened at their side are the ones where that virgin Queen was killed by the Queen which emerged first or killed by the bees in case they don't need her.

I also found in most bee books and net that a QC opened at the end tip indicates that a young Virgin Queen has emerged. But this is not entirely so :)

In one of my nucleus hives I assumed after seeing the QC being destroyed/opened at its side that they have killed the young queen and I gave them another comb with fresh eggs but as it turned out they already had a Virgin Queen.

I was puzzled until I went to my beekeeping friend last week and looked at his Queen Mating stations. He breeds Queens in small cassettes which have glass walls on two sides for easy inspection and MOST of the Queen Cells were opened at their sides !!!
I don't get it! Why would worker bees release the queen through the side wall which is much thicker than the thin end tip wall???

This experience have shown that bees can release their virgin queen in both way; through the end tip and through the side wall.

This was a great lesson learned for me so I felt to share it.