I'm not sure there's much to explain here except that the writer made a mistake. The correct sentence would use your alternative: If it were gentle, only the more unstable rods would fall. If it were severe, they would all fall. Another possibility would be a mixed conditional, which mixes a present condition with a hypothetical past result: If it were gentle, only the more unstable rods would have fallen. If it were severe, they would all have fallen (actually, I'm not even sure what this sentence even means...I'm assuming “it" refers to wind, that's the only "it" word I can think of that collocates with "gentle" and “severe”.)