The numbers there are for extra attacks depending on level. A 1 means +1 attack, a -1 means +1/2 attack (weird use of negative numbers). So it is why it goes weirdly from 0 to -1 to 1 to -2.
The original file by the way just goes up to -2 which is somehow wrong with AD&D rules I was told (hence the level 50 patch to rules that gets this modifier up to -3 which is 2.5 extrac attacks).
I'm actually wondering where this file comes from though since it has weird jumps from -2 to -3 without going through 2...

In PSTEE we have:

2DA V1.0
0
    LEVEL1  2   3   4   5   6   7   8   9   10  11  12  13  14  15  16  17  18  19  20
0   0       0   0   0   0   0   -1  -1  -1  -1  -1  -1  1   1   1   1   1   1   1   1
1   0       0   0   0   0   0   -1  -1  -1  -1  -1  -1  1   1   1   1   1   1   1   1
2   -1      -1  -1  -1  -1  -1  1   1   1   1   1   1   -2  -2  -2  -2  -2  -2  -2  -2
3   -1      -1  -1  -1  -1  -1  1   1   1   1   1   1   -2  -2  -2  -2  -2  -2  -2  -2
4   -1      -1  -1  -1  -1  -1  1   1   1   1   1   1   -2  -2  -2  -2  -2  -2  -2  -2
5   -2      -2  -2  -2  -2  -2  2   2   2   2   2   2   -3  -3  -3  -3  -3  -3  -3  -3