it is not confusing, it is the average number of members who play each event.
no matter how many guy you really have in your team.
example : in a team of 40 members
event 1 : 35 member did play
event 2 : 36 member did play
event 3 : 30 member did play
event 4 : 38 member did play
event 5 : 33 member did play
so your team member average gonna be : 35+36+30+38+33 =172 divided by 5= 34.4 witch will put your team in the HW division.
an other example:
since it is an average, if your team member average is let's say 22 after 9 events, you will be able to play the last event with only 12 guy and still keep your team in the LHW division as shown here :
22+22+22+22+22+22+22+22+22+12=210 divided by 10 = 21 so still ranked in LHW even if your were only 12 at the end.