It's a good question. Here's the answer we've found, and we promise that this isn't an early April Fools...
The fact that the different component scents in a perfume are called 'notes' seems to be traced back to one of the very many attempts to classify and organise the complicated world of olfaction. As outlined in the introduction of Jonathan Reinarz' book, 'Past Scents - Historical Perspectives on Smell (2014)', a British chemist and optician, named George William Septimus Piesse, attempted to 'translate' music into smell - and back again. George created an 'odophone', or 'smell organ', which consisted of perfume-containing atomisers, which were activated by piano keys. The 'odophone' released heavy odours to correspond with low music notes and sharp odours to correspond with the highest pitched music notes. A mixture of the played 'scents' was referred to as a bouquet and a good perfume was coined by him as one that was 'in tune'.
Although this curious musical instrument is now long-forgotten by many, his concept has endured and in modern perfumery today, different odours are still referred to as 'notes' and a perfumer's work station is still called an 'organ'.
So now you know! Stick this one up your sleeve for a future pub quiz.