What does the first para of Lonaphala Sutta mean?
This is how I understand it.  You don't get back exactly what you put in, but you get back the results of it. 
Example,  if you kill someone, it is not necessary that you will get killed back in return but you will get results from your killing which depends on many factors.  This sutta,  Buddha taught us that is possible to work on those many factors that could produce a better results.
Like , if I killed someone, then later worked to become a stream enter, a result that could have sent me to hell next life changed to make me a sickly person.  Etc. A Finger bandit arahat is another example.
It is clear to me that karma is not fate or fixed destinations or replacement of a will from a god.