Offsetting gives you a little extra space in only one of the directions (vertical). I should probably change this to be a more optimal packing, but you can see the effect pretty clearly if you just load a pure black or pure white image, then play with the different settings at really low res. For example:
Without offsetting odd lines:
With offsetting odd lines:
From these pics, it should be very clear that a 5mm spacing with a 5mm hole still has the holes for a single row touching, but between rows there's a little more space with the offset enabled.