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.