Ok, we're all arguing middle school math here, however the simple fact is that if you're gaining muscle mass, the scale can stay the same, but your clothes will get looser.
I ran The Goofy at WDW in January. This means that I ran a half marathon and a full marathon (39.3 miles in total) in two days. Training for this was the most physically intensive thing I've ever done, and it really took me a year to get from the point where I decided I would do it, to the point where I COULD do it. I gained 12 lbs during that year. I also dropped from a size 6 to a size 4. This is because I replaced my less dense fat with more dense muscle. And I gained weight, despite becoming smaller.
Ok, so I was 11 weeks pregnant when I ran the Goofy. I am now 24 weeks pregnant. I have only gained 4 lbs total since I got pregnant - my OB was worried until she saw my ultrasound results. My baby is actually above average (by about 2 weeks) in size, despite the fact that I have gained minimal weight during my pregnancy. How? I am no longer running 50 miles a week so my more dense muscle mass is now being converted to less dense fat/water/baby. I have gained a deceptively small amount of weight, because I am simply losing muscle mass at the same time that I am gaining baby weight. Makes sense?
This is why the scale can be a good tool for weight loss, but it is not the be all, end all. You will have fluctuations and you will have drops and you will have gains... but I think that your clothing size and how you feel is a better gauge of how you're doing.