Quick answer
A free internal event will always have a no-show gap. Measure it by checking people in, plan the next event against your own measured rate, and stop treating registrations as attendance.
Three numbers, routinely confused
Invited, responded and attended are three different things, and the error compounds when they are reported as one. A room booked against registrations and catered against invitations will be wrong in both directions at once.
Cause 1: free events have no commitment cost
When saying yes costs nothing, some yeses are aspirational. That is not cynicism, it is a property of internal events — and it means your no-show rate is a number to measure rather than an anomaly to be annoyed by.
Cause 2: the invitation went to a distribution list
A message to all-staff@ gets a reply from whoever reads all-staff@. Individual invitations produce individual accountability, and they let you see who has not answered — which a distribution list structurally cannot.
Cause 3: nobody was reminded
A colleague who accepted six weeks ago has forgotten. A reminder the day before, with the room and the start time, is the cheapest attendance improvement available to any organiser.
Cause 4: sessions were counted as one event
If your day has three sessions, you have three headcounts. Treating each session as its own part gives you numbers per room rather than an average that fits none of them — which is the difference between a keynote with empty rows and a workshop with people standing.
Cause 5: nobody checked anyone in
Without a check-in step you never learn your real attendance, which means next year's planning starts from the same guess. Checking guests in from a phone at the door takes seconds per person and converts a guess into a measurement.
Plan against your own rate
Published industry no-show figures are close to useless for a specific company. Two or three of your own events give you a rate that reflects your culture, your calendar and your building — and it is usually more stable than people expect.
What to tell finance
Report all three numbers, every time: invited, confirmed, attended. It makes the catering decision defensible, it makes the following year's budget an argument from evidence, and it stops the conversation where somebody asks why 200 lunches were ordered for 140 people.