In Heap, a graph and a funnel are measuring two different things. While a graph is measuring an event count, a funnel is measuring the number of unique users who completed the action.
For example, if six users clicked the ‘sign up’ button once, and two users clicked the ‘sign up’ button twice, the graph would show a count of 10 and the funnel would show a count of 8. The funnel also shows the user count over the entire time range, where a graph shows the totals per day or week even if unique users is checked.