Kammas are imponderable? Buddha says if one plants mango, he will reap only mango not something else. Short-lived and get killed are the result of killing living beings, poverty is the result of stealing, not trusted by others is the result of lying and so forth. Then why Buddha says the result of kamma is one of the four imponderables. Does it mean one who loves to kill living beings will experience poverty or not trusted by others which is the result of stealing and lying? How do we relate that with "sow apple seed, reap apple fruit?"
The kammas are imponderable because people tends to expect action 'A' resulting in the equal fruit 'A'. However as we know a single action 'A' is only one cause, there are many other conditions (other causes) which result in a fruit. Just as a cup of coffee implies the existent of coffee beans, but coffee beans by themselves do not necessarily result in a cup of coffee. There are so many other factors such as farmers, the sun, customers wanting to drink coffee for the outcome 'a cup of coffee' to result.

In my temple we like to say that the merit of a good deed is 'immeasurable'. Why is this? Because the outcome cannot be entirely predicted, a single good action, can lead to opportunities for further good actions to occur. For example, you help someone with a small task, and then that person gives you an opportunity to do more good, and eventually eventhough the original good action was only the beginning seed the final result is immense. 

The only thing we know for sure is that good actions will result in good outcomes (if something consistently results in bad outcomes it is a bad action and you should stop doing it). So instead of trying to predict the final outcome, you should focus on the simple strategy of doing more good as opportunity allows.

Addendum regarding the comment

My understanding is that because the Four Imponderables can not be deduced logically therefore it is imponderable. For example a common stated effect of karma is said that killing leads to a shortened life span. There is no clear logical explanation why this is the case, you could argue it's through some kind of revenge, or perhaps susceptibility to illness caused by psychological damage, but it's only speculation.

Likewise, releasing animals doomed for butchering are said to be good karma for longevity, but once again there is no logical explanation why it would work.

Since speculations about the exact working of karma does nothing to lead to peace and end of suffering, the Buddha ask you to refrain from pondering about them and take it at face value.