Real musicians do not generally play exactly in time: they play with 'feel', where there can be 'microshifts' in timing. The snare might be slightly late on beats 2 and 4 ('laid back'), while the kick or the bass might be slightly early or 'pushed'. The overall rhythmic pattern might rush or drag over several beats or bars, giving life to what could otherwise seem too rigid and stiff. Fortunately, most DAWs (and certainly Logic) offer ways to help you preserve or impose this kind of timing character.
http://www.soundonsound.com/sos/nov08/articles/logicworkshop.htm