It looks like the formula just changed again. For the record, I liked the last formula a whole lot more.
My best instrument was Harmony, because that's where I had the highest skill. Now my "most played" instrument is Guitar, because I've played the most songs on that one. And yet, only a few more than what I've done on Vocals (well within one standard deviation), which again has a much higher skill rating. But yes, technically Guitar is my most played instrument. Maybe some weight should be given to percent of songs played, where the denominator consists of songs that even have charting for each instrument. That would give Pro instruments a chance to appear here, since relatively few songs can be played on them.
My best song used to be "Paradise By The Dashboard Lights", I'm guessing because that's where I had my highest band score (in terms of total points). Now, it's "Need You Tonight", probably because that's my highest single-instrument percentile rating (99.7% on Keys / Pro Keys). The move from points to percentile is a good one, but I thought it made more sense when it used Full Band score. Either way, weight should be given to the song difficulty (much like how it is done for Signature Performance on this site, even if it doesn't use the exact same formula).