Shared libraries

Precisely this - most likely you are only loading your process executable in memory, all its dependencies are already paged in. It's not something you noticed on an x86 desktop with fast storage, but it's definitely something you notice on a resource strapped arm machine with a crappy emmc. And even on x86, it's definitely something you notice when the resources you are using are resources you can't give to paying customers (eg: a virtualization host). At that point, running a bunch of static linked massive binaries and all the extra memory they take starts actually costing you money.