a_2     2
a2      1
[download]

$ sort -k 1,1 test.txt | cut -f 1
a2
a_2
[download]

a_2
a2
[download]

Looks like two level sort: on the first level, it ignores the _, but if the strings are different only thanks to the underscore, it is used in the second level.

$ echo $'a_2\t2\na2\t1' | sort -k1
a2      1
a_2     2
$ echo $'a_2\t2\na2\t1' | sort -k1,1
a2      1
a_2     2
$ echo $'a2\t2\na_2\t1' | sort -k1,1
a2      2
a_2     1
$ echo $'a2\t2\na_2\t1' | sort -k1
a_2     1
a2      2
[download]