How to understand iOS memory physical footprint

Question

According to this WWDC iOS Memory Deep Dive talk https://developer.apple.com/videos/play/wwdc2018/416, memory footprint equals to dirty and swapped size combined. However, when I use vmmap -summary [mempgraph] to inspect my memgraphs, many times they don't add up. In this particular case vmmap , memory footprint = 118.5M while the dirty size is 123.3M. How can footprint be smaller than the dirty size?

In the same WWDC talk, it's mentioned that heap --sortBySize [memgraph] can be used to inspect heap allocations, and I see from my memgraph that the heap size is about 5M All zones: 110206 nodes (55357344 bytes) , which is much smaller than the MALLOC regions in the vmmap result. Doesn't malloc allocate spaces in the heap?