Does the execution of threads differ at every execution?

Quote:

Is this they way it is supposed to be?

Yes.

Ah, threads ... are they the same ever time?

In an ideal situation every app will always be deterministic: each time you run it, it will give the same results. But we live in the real world, and our OS is not doing the same things each time your app is run. So when you start multithreading, you have to realise that each thread is independent unless you specifically code for them to synchronise (and that gets extremely complicated if you aren't very, very careful). That independence means that the OS is at liberty to run them as it sees fit, and as cores withing your processor become free or are preemptively freed to allow other threads to run.

So depending on how you have coded them - and it doesn't sound like you have coded them as independent units, they seem to be accessing the same variables and that's very dangerous - you can get very different results on two separate runs. Without your code, we can't tell what might happen in your specific case, but do be aware that when you write a line of code, no matter how simple it is it isn't executed as a single unit, it is executed as a set of steps, so if another thread is also accessing the same variables, the results can become very unpredictable because they can literally be changing the same variable at the same time!

This is not a good field for beginners: you need to think very, very carefully and plan out threading before you start coding, or you can get into the situation where one thread is affecting another and they both cannot run until the other has.