The (non-numeric values) bucket appears when some of your event data has values for that property that were sent as a non-numeric data type (e.g., strings like "N/A", "unknown", "", or other text) rather than actual numbers.
In other words, even though your "number of orders placed" property is intended to be numeric, some events were tracked with a value that Mixpanel cannot interpret as a number. [Custom Buckets FAQ]
Here's the distinction between the three categories you're seeing: