For the ladies, your cycle can make a big difference.
Say you lose 1.5 lbs a week, but on your * week you gain 2 lbs of water.weight. The scales will say you've gained, even though you will have done nothing different.
I gain water round ovulation too, so again, gaining 1lb of water will mean you only show 1/2 lost.
I have a tendancy to gain water if I eat salty foods too.
Sometimes its better to see what your average is over a few weeks to iron out these discrepancies.