Why vmmap shows that footprint is less than dirty size

I use vmmap check my app’s memory,find that dirty memory is bigger than footprint ,the physical footprint is 1.8 g, but dirty size is 2.3g,why?

The total dirty size is 2.3G and the total swapped size is 686kb, physical footprint is 1.8G,physical footprint peak is2.3G,I see some articles said that footprint=dirty +swapped,why my app shows that dirty size is more than footprint?anyone can help me ?

Why vmmap shows that footprint is less than dirty size
 
 
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