I think you've largely answered your own question. The strips are quite sensitive to water intake. They are also very sensitive to your very recent activities - like over the past several hours - so if you did things differently on that night than other nights that can account fo a significant swing. Also, do you normally test at night. If not, there's another huge variable. It is best to test first thing in the morning as that places a fairly consistent six to eight hours that are the same leading up from test to test.

Hope that helps.