This is due to discord limiting the widget to 100 members, its not good practice using the widget to get online members, a better solution would be to use a bot and create a small API to get the online members.
This is what the presence_count field is for, it's all of the non-offline members, currently I assume the module checks the size of the members table rather than just using presence_count - I'll let Chute know that this needs changing when he has the chance