I wish I knew to be honest..I suppose if you have not drunk enough water and you know this, and your weight was low, then you can up your water intake and see what the result is the following week.
LT say an average of 1 stone a month, but there are so many factors to build in, TOTM, whether you are retaining water, etc.
I dont know whether you can say why sometimes. There are people who have stuck 100%, drank sufficient water, exercised and not lost a lot one week, and then the other, they have!
I suppose the scales dont really know it is your weigh in day, so you could quite easily lose a whole pound the next day after your weigh in!! Who knows.
So long as you see your weight coming down each week that is the main thing.
Maybe others will have more insight and inspiration!
Hope you are having a good week.
Take care