The amount of bad data on last.fm and their lack of interest in cleaning it up makes me sad. You can see a small example just by looking at the current top songs: http://www.last.fm/charts/track Morning Mr Magpie is in position 8 instead of position 4 where it should be, because 8105 people scrobbled Morning Mr. Magpie with a dot. Tip of the iceberg. Really egregious are the albums with no scrobbles because every tra…
http://www.last.fm/api/show?service=446 http://www.last.fm/api/show?service=447
What you're probably seeing there is tracks which are so new that we've not determined similar fingerprints for them yet, or those changes have not propagated to the charts (we use a lot of caching as you might imagine). In fact, checking the chart now it seems that that particular problem has cleared up.
Also, artist disambiguation is a _hard_ problem, but it's something that we'd very much like to address.