Why are two similar bits of code giving different results?

[Kebap]

Which comes first?
Does they even change a value, or just after the next cycle?

No single value comes first. First all three values on the right side are calculated, only then the original values are replaced. So, yes just after the line, all values are changed at the same time. This is an easier way to write it without storing extra variables. Consider this:

a, b = b, a

When you want to split it in multiple lines, you need a third value c. Try it, maybe it's easier.