I just cleaned my chimney, so why did I still have a chimney fire?

I have a large wood boiler in my garage. It has a 20-ft long, insulated stainless steel chimney. A few days ago I cleaned it with a chimney brush as I do every few weeks throughout the winter. This gets rid of most of the creosote, but there is always a thin sheen of the hard, shiny black stuff left on the inner wall - it has always been there and I'm fairly certain it's impossible to get rid of all the creosote. For some reason last night I had a chimney fire. It was scary but burned itself out after ~15 minutes (I blocked the chimney at the bottom). I've never had a chimney fire before I umed because I was doing a good job cleaning it. There will always be some creosote in the chimney, so I ume I'm not the only one in the same situation. Was it a fluke? Perhaps I should be cleaning the chimney all the way down to the bare steel every now and then?