comm should tell you that one of the files isn’t sorted:

comm: file 1 is not in sorted order

To compare the files, you can pre-sort them (lexicographically as you point out):

comm <(sort file1) <(sort file2)

If you want the result to be sorted numerically, sort it again:

comm <(sort file1) <(sort file2) | sort -n

This produces

1
2
3
4
5
6
7
8
9
11
12
13
15
        16
17
        18
19
20
        21
22
    23
    705
    707
    709
    711
    712
    826
    827
    839
    846
    847
    848
    872
    873
    874
    875
    891

comm should tell you that one of the files isn’t sorted:

comm: file 1 is not in sorted order

It expects the files to be sorted using the current locale’s collation order (as determined by LC_COLLATE); it won’t accept numerical order.

To compare the files, you can pre-sort them (lexicographically as you point out):

comm <(sort file1) <(sort file2)

If you want the result to be sorted numerically, sort it again:

comm <(sort file1) <(sort file2) | sort -n

This produces

1
2
3
4
5
6
7
8
9
11
12
13
15
        16
17
        18
19
20
        21
22
    23
    705
    707
    709
    711
    712
    826
    827
    839
    846
    847
    848
    872
    873
    874
    875
    891

comm should tell you that one of the files isn’t sorted:

comm: file 1 is not in sorted order

To compare the files, you can pre-sort them (lexicographically as you point out):

comm <(sort file1) <(sort file2)

If you want the result to be sorted numerically, sort it again:

comm <(sort file1) <(sort file2) | sort -n

comm should tell you that one of the files isn’t sorted:

comm: file 1 is not in sorted order

To compare the files, you can pre-sort them (lexicographically as you point out):

comm <(sort file1) <(sort file2)

If you want the result to be sorted numerically, sort it again:

comm <(sort file1) <(sort file2) | sort -n

This produces

1
2
3
4
5
6
7
8
9
11
12
13
15
        16
17
        18
19
20
        21
22
    23
    705
    707
    709
    711
    712
    826
    827
    839
    846
    847
    848
    872
    873
    874
    875
    891