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 BG2EE 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 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
0    0    0  0  0  0  0 -1 -1 -1 -1 -1 -1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  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  1  1  1  1  1  1  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 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -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 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -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 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2
5    1    1  1  1  1  1  -2 -2 -2 -2 -2 -2 2  2  2  2  2  2  2  2  2  2  2  2  2  2  2  2  2  2  2  2  2  2  2  2  2  2  2  2