<p>Its not that I an unable to understand the difference, but if you read something really fast (more than 400wpm), the natural tendency of the your brain would be to catch only those components which it is familiar with. Since I am not very familiar with ‘not uncommon’ and more familiar with ‘common’, is the reason why we had different experiences using the two ‘controversial words’. But yeah, if you go according to general use of language, it (‘not uncommon’) might be turned as a flaw, but still it is grammatically correct.</p>