Your weight flucuates about 5 pounds per day, depending on how much water you drink/don't drink, sodium intake, etc. In our weight range, it may even be a little more, since we have more water and therefor more room for dramatic swings. There are times because of my medical problems, that I have to weigh myself morning & night daily- its amazing to see how much the numbers can jump around in just 12 hours. If I don't get enough water, I can see it in the scales as my body starts conserving all the water it can and my weight goes up a couple pounds. When I up my intake again, it goes back down pretty quickly, which is the opposite of what most people would think.
As far as weight loss tracking, its probably best to pick one day/time a week that will be your official weigh in- like with your Weight Watchers meeting, even if you weigh yourself daily. Some people suggest that if you weigh every day, then you should take the average of the weights as your 'official' weight for the week, which makes sense. Losign weight is not a race, its not a steady downward path- it takes time and you will see ups & downs within your weight, even over just a few days. Personally, I do tend to weigh myself most days- I don't neccessarily write it down or anything, but just watch that I don't go more then 4-5 pounds above my lowest number...if I do, then I know I'm not doing something right and need to get back on track. I know for me that if I start avoiding the scale that I'm probably off track and in denial. The scale helps keep me focused and on program, but I don't let the number on the scale ruin my day- its just a number that can help guide me.

If you had heavy winter clothes and shoes, it can make a big difference too. The best way to compare scales is to weigh yourself wearing the same clothes/shoes right before you leave or as soon as you get home- and try to not eat, drink, use the bathroom, etc between the two weigh ins. I've done that in the past to see how accurate my scales are compared to my doctors office- and they are pretty good. Its also helped me to realize its okay that if the doctors scales say I weigh 4#'s more then what I weighed this morning at home, because I probably do by that time in the day with all my clothes on.