Claiming Assignments Limited to one person - however overlap is not prevented


We have had a strange occurrence (a few times but not regularly). The column is limited to one team member claiming the assignment. All parties involved were using a browser on a PC. The first person to claim the assignment (green) claimed it the prior day. The second person (blue) saw that it was empty and went to claim the assignment - not at the same time. Is there anything we should be doing differently or is this just an occasional glitch?

Thank you in advance!