How does duolingo decide when to give me new real world examples?

[ajkojjls]

I sometimes give comparatively good answers and the programm suggests to them once more while with others (of which I know that I have done a rather poor job) do not show up again. Maybe I have just a wrong perception?!

[rehana]

I thought it wasn't supposed to give you the same sentences twice, but I could be wrong. (Not the same as giving you sentences 4-6 when you've done 1-3)