It happens when someone else paid for the same last item before you. Keeping in your shopping cart does not mean you are owning it. The system prefers someone who goes first. The system does not work like the physical shopping cart at your local supermarket. If someone took your item in your shopping cart there, you can stop him or her, but in the virtual shopping cart, it does not happen and the system use the basic rule, whoever pays first, get the last item first. Unfortunately, we can not do anything about this.
Thank you for understanding.
CSTOYS International