Thank you for fast reply and warm welcome.
Actually all albums (12 flac albums right now) in my library have the same situation. I have checked one of the affected albums in my library and the result is as shown in below.
I am not the expert for the bitrates therefore I might need some help in here. I am not sure what should be the expected value.
same is also true for ripped cd. The cd had been ripped as a single flac then splitted into tracks with cuetools and this is the result.