Ultimately, RAM is ONLY guaranteed in the form sold (from the exact same pack), as the manufacturing process is delicate, and changes frequently, so technically you could get 2 of the EXACT same RAM model, but from 2 different packs, and they might not work together.
Not saying it never works, but it's a 50/50 gamble, and the only way to know is to try the RAM together.
Getting RAM that is the same timings/voltages etc. is just a way of minimising risk of conflict, but by no means a way of eliminating it.
It's why best practice says you should always buy your RAM from the exact same pack, but there are also plenty who have mixed modules working fine. It's just a gamble.