The counter of participants always show all participants of the course, but the assigment is restricted to a group and I think the number of group members should be displayed. There are also all participants in the grading list, regardless if they are in the group of the assignment. So people, who do not see the assignment, may be graded.
Steps to reproduce:
Use an empty course with one manager without a group.
Create two groups and two suitable groupings, where you put one group in.
Assign two students to the course and put one in the first, other in the second group
Create an assignment, take in field "Grouping" one of the groupings, check "Available for group members only". The field "Group mode" may be set to "No Groups", "Visible Groups" or "Seperate Groups", every time the count is wrong.
Save and click your assignment and you will see in the grading summary: Participants 2
But the assignment is only for the one student in the Grouping, choosen at the setting "Grouing". Also in the grading list both students are visible, but only one can really submit, the other one doesn´t see this assigment.
I tested this in Moodle2.3.2+ (Build: 20120920) and Moodle 2.5.2 (Build: 20130909), both have the same problem.
It is possible to get a list only of one group, when i choose "Serated groups" in the field "Group mode" and select the group in the dropdown menu manually, but also there the default is "All participants" where user are counted not belonging to this assignment and they may also be graded.
I think thats a bug, what do you think? In Moodle 1.9 only group members are shown (a count doesn´t exist there