Hey did they do this on Purpose?? Okay Tsumugi Kotobuki shares the same last name as her Voice Actress, Minako Kotobuki. This is sort of Odd
If only Mugi got her own song in K-on!! this is sang by the band Sphere which include two of the Voice actress from K-on!!
So two of the Voice Actress of K-on!! are in a Band called Sphere? (Yui & mugi's voice)
Cover of there album