Because the figure is what you kept, after refunds. A refund on a wheel order comes off the revenue when you make it, and an order that is cancelled, fully refunded or whose payment fails comes off entirely and leaves the order count. If that order is paid again later, what is left of it is added back once. A refund you delete is put back, unless the order is still marked Refunded or Cancelled; then it comes back when you move the order to Processing or Completed.
This counts from the update that added it. Older refunds stay in the figure until something new happens to the same order; then that order catches up with all its refunds at once, which is why the first drop can be bigger than the refund you just made. On a shop that had the wheel before, the Revenue card says the day this started: after refunds since that date.
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