That's a good question. It's because you're looking at the nested for loops wrong not the 2D array.
Think of nested for loops like this:
Run for loop #1
Run for loop #2
Is for loop #2 done? Yes then move back to for loop #1. No then execute for loop #2 again.
And it keeps going like that.
So the equations would look like this in order:
nums[0][0] = (0*4) + 0 + 1
= 1
nums[0][1] = (0*4) + 1 + 1
= 2 - because for loop #2 isn't done "t" stays at 0 while "i" increments by 1
nums[0][2] = (0*4) + 2 + 1
= 3 - again, for loop #2 isn't finished
nums[0][3] = (0*4) + 3 + 1
= 4 - same again
nums[1][0] = (1*4) + 0 + 1
= 5 - finally for loop #2 is finished so now loop back to for loop #1 and increment "t" by 1. For loop #2 restarts and therefore "i" is reset to 0.
nums[1][1] = (1*4) + 1 + 1
= 6 - and it starts again etc.
Hope that explains it. If not I'll try again
.