Hallo Gao,
look also in chapter 2.6 of the manual. There is explained in more detail which bands are used with which resolution. Consider the similarity of band and band 8a. They are located at the same spectral position, however band 8 is at 10 m resolution and therefore requires a broader spectral halfwidth It makes little sense resampling this band to 20 m or 60 m when a channel with a smaller bandwidth is available for the same spectral position in that spatial resolution.
Regards
bdpg