The limit of the characters shown in your guild member list is 500. So there'll be some of them who won't be displayed in the list when you have more than 500 characters in your guild.
Carto_guildpositions is not working properly when you are in a big guild like this. I guess this is because the addon can't get information of some people who are not in the list via guild list.
Hope this can be fixed. At least don't make the addon report error when you move your mouse over a guildie who is not in guild list.