1024px is the max screen width it’s aiming at. We need to allow some window chrome, so it needs to be less. We’d ideally like it to have lots of factors, allowing us to split it into equal size columns with integer widths.
960 has lots of factors:
echo factors(960);
1 2 3 4 5 6 8 10 12 15 16 20 24 30 32 40 48 60 64 80 96 120 160 192 240 320 480 960
1000 doesn’t have as many
echo factors(1000);
1 2 4 5 8 10 20 25 40 50 100 125 200 250 500 1000
Specifically, you can easily split 960 into 2,3,4,5,6 and 8 columns.