The difference between a count and a confirmed count
Any estimate feels solid until the day. A confirmed count is different because each unit of it is a guest who answered on a page, attached to a name, and totalled in front of you. When Elin gives the hall a figure, she is giving a number the list built, not a number she hopes is close.
That distinction is what an attendance confirmation website is for: turning hopeful estimates into a settled count weeks early, keeping it true through late changes, and finishing with a door that knows who arrived. Everything else the event needs can wait; the deadline cannot.