RAM usage shown means little with out understanding the context.
If you run Activity Monitor and process X is using 8 GB and Process Y is using 3 GB it just means those process X has X page of memory allocated to it and process Y has 3 GB of memory pages allocated to it. And all of that means little since we don't know if any of that memory is critical to the processes (i.e does the process need that memory to do it's work, or was the memory just allocated and deallocated, but the system has not yet given it to another process.)
To see that you need to look at "Memory pressure", the conditions there (green, yellow, red) show if that memory is really critical to those processes. If you start seeing yellow then you are running short of RAM and are headed toward an issue. It is called trashing. When Process X is having to get memory from Process Y to do some work, and then Process Y has to get memory from Process X to do some work. If it is Red you have a critical issue and processes are starting to stall for lack of memory because they spending all their time having to give and take memory from each other rather than doing real work.
As to why it is different on two machines. Because unless you can create the exact same condition on both machines. Exact same running processes, exact same order of operations in those processes, exact same resource (hard drivers, network, etc) they will be different because they got to where they are along different conditions.
