If you're just scooping with a teaspoon and not trying to be exact, there will be some variation in the amount of leaves you use from cup to cup. This could result in an extra cup or two. Plus I think somewhere around here they said that the actual amount of leaves in the tins vary a little. It's supposed to be about 1oz, but maybe you'll have more than that, depending on the size of the leaves going into the tin. I wouldn't worry about it too much, especially if you're on pace to get more cups than you expected. If you were going to get significantly less than expected, then I'd be worried if I was doing something wrong. I think you're fine.
klemptor, you might have said something like this in response to my recent question in another thread. I use a regular dining teaspoon because I thought I read/heard that it was designed not to hold a measuring teaspoon, but to hold the right amount of tea leaves for a cup of tea. *shrug* I think a measuring teaspoon would hold less leaves, I'm not sure though, and I might get a weaker brew than I'm used to, so I'll probably stick with the dining teaspoon.