It's likely contradictory for a reason. If it was titled something that suggested it was about lack of rain, like 'Drought' for instance, we'd know what was coming and it wouldn't have much of a impact. By calling it 'Rainfall' and describing a lack of rain, it makes you stop and notice the contradiction, and thus the poem occupies more time in your active memory and makes a more lasting impression - which is particularly important for such a short piece.