I'm having trouble using the lookup function.

I have numbers that look like this in a list, although it's only part of
what I'm using:

-11 2179
-10 2420
-9 2661
-8 2897
-7 3123
-6 3332
-5 3521
-4 3683
-3 3814
-2 3910
-1 3970
0 3989
1 3970
2 3910
3 3814
4 3683
5 3521
6 3332
7 3123
8 2897
9 2661
10 2420
11 2179

When I use the lookup function on another sheet of the cell, it does not
look up the proper value always. My columns go from (-100, 100). Outside
the values of (-10,10), the lookup works properly.

These are the lookup cells:

-11 2179
-10 2420
-9 2420
-8 2897
-7 3123
-6 3332
-5 3521
-4 3521
-3 3814
-2 3814
-1 3910
0 3989
1 3989
2 3910
3 3910
4 3683
5 3521
6 3521
7 3123
8 3123
9 2661
10 2420
11 2179

If you'll notice, the values that the lookup cells get are not the values of
the cell they're looking up, but often one off.

Any suggestions on why this might be happening?