Sometimes your progress towards a challenge won't match the monthly total you see on the leaderboard. This is expected, and there are two straightforward reasons for it.
Only activity Smyles count towards challenges
Challenges are all about physical activity, so they only count Smyles you've earned from recording activities. Smyles earned in other ways, such as completing a challenge or trophy, or being sent a high five, don't count towards your challenge progress. They'll still show on your overall totals, which is why the two figures can differ.
Challenges only count from the moment you join
A challenge starts counting your Smyles from the point you join it, not from when the challenge itself began. If you join a few days into the month, any Smyles you earned before joining won't be included in your challenge progress, even though they still appear on the leaderboard.
So if your challenge total looks a little behind your leaderboard total, it's usually one of these two things. To get the most out of a challenge, it's best to join it as early as you can, so every bit of activity counts from the start.