Steps to reproduce:
1) create/use a course with at least two groups of student
2) add new Choice activity with following setting:

• Limit the number of responses allowed = Enable
• Choice 1: Choice = First Choice ; Limit = 1
• Choice 2: Choice = Second Choice ; Limit = 1
• Group mode = Separate groups
  3) Login as a student from first group and make a choice
  4) Login as a student from another group and view the choice

There is a problem: the choice made by a student from another group is taken into account and it makes the choice item (Full). IMHO this is a bug in Separate groups mode. Only choices made by students from the same group should count.
See the screenshot attached: even the choice has not been answered yet in the current group, the First Choice is already full...