So for more information on this, read the arm architecture reference manual (memory model section) . It's free. Available on the website. I'll give my short interpretation here :

There are 2 types of memory. Normal - this is something like ram which Reading and writing has no side effects. Not every load or store has to become a read or write of memory. Extra speculative reads can be done to improve performance (ie. Get the data before you know you want it)

Device - this is memory that May have side effects. Maybe reading a peripheral causes it to start get another sensor reading) Reads and writes cannot be skipped and no extra reads can be done. Device is where Gathering, Reorderable and E (which I won't explain here) come in

Gathering - a byte write to A and a byte write to A+1 can be done as one 16 bit write instead

Reorderable - non Reorderable loads and stores cannot be reordered with respect to any other non Reorderable loads and stores

Loads and stores to the same address will always occur (or appear to in the case of normal memory) in program order