iOS - XCode debug

while i'm debugging my app there is some strange things. In some point the code line isn't be debugged one under another. Example :

LINE 1
LINE 2
LINE 3

Sometimes LINE 3 is debugged and then Xcode return to line 1 and proceed normally with line 1, 2 and 3. I hope i explained the problem well. Thanks

One possible explanation is that optimising compilers re-order code to improve cache coherency, avoid pipeline stalls, and other arcana.