first block in ext3 filesystem

Hi, I came across this $sudo dumpe2fs /dev/sda1 4KB Block Size Group 0: (Blocks 0-32767) Primary superblock at 0, Group descriptors at 1-1 1KB Block Size Group 0: (Blocks 1-8192) Primary superblock at 1, Group descriptors at 2-2 Why does first block start from 0 in the 4KB block size partition and 1 in 1KB block size partition ? Regards.