Unusual versus Not Unusual

It totally occurred to me today, while listening to the cross examination of a company director accused of VAT fraud, that 'unusual' is not the opposite of 'not unusual'. In fact, both phrases are often used to mean the same thing. The cross-examiner thought he had his witness well and truly stitched up, however. He'd just said aloud that it was 'not unusual' for phones to be shipped before the proper paperwork was filled in, whilst in his witness statement he said that the incident in question was 'unusual'. In both cases, he clearly meant that such things happened, though not frequently. The witness struggled to explain the apparent contradiction, which is what happens, of course, if you don't think carefully about the stock phrases you reach for almost instinctively.