Why are deleted group members getting group items?

Group Item (like a Group Action or a Group Metric) always has two parts: the Group Item itself and the respective individual items, like the individual client Actions for each member of the group who's part of that Group Action.

When you remove a Group Member, they are no longer part of the group item BUT they still have any corresponding individual items -- CoachAccountable is polite, perhaps to a fault, to not delete anything that potentially should be left there.  

This is often nice because say, for example, someone's part of a Group Metric and are tracking their data for a while, and then you remove them from the Group: that person might still like to have that data that they tracked.

So if there are group items someone WAS participating in before you remove them from a group and those items shouldn't be around post-group membership, you or your client will need to manually delete those respective individual items.

There's one potentially useful caveat to this, and that is how the system treats the act of modifying which members are included in a Group Item (this is done by clicking the Edit icon for a given Group Item, found in the bottom right corner of the item on the Group Stream tab):
When you uncheck the box for a given group member to have that person be no longer included in that specific Group Item, CoachAccountable WILL delete the respective individual item, BUT ONLY if there's nothing yet been done with that item.
If the individual's Action has been marked done, for example, or Metric has one or more data points recorded, or Worksheet has been worked on or completed, CoachAccountable again (politely) figures "Okay, that person has done something here so we'll let them keep it".

This all suggests that, when removing members from a group, you might find it best to selectively remove them from specific Group Items (in order to delete the right respective individual items) and THEN remove them outright from the group.