In Heap, a usage over time chart and a funnel measure two different things.
While a usage over time chart is measuring an event count, a funnel is measuring the number of unique users who completed the action. For example, if eight users clicked the ‘sign up’ button once, and one user clicked the ‘sign up’ button twice, then the usage over time chart 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 usage over time chart shows the totals per day or week even if unique users is checked.
For more information about funnel analysis, see our Funnel analysis overview guide.