Fitbit 16K steps BodyMedia <8K.??!

First day back to wearing my body media fit core. I have been reaching my goal of 10K steps daily until today...well depending on which device I wanna listen to. First of all, I walked almost 5mi on a pre measured road, and so I know I should have at least 10K. And then I was very busy today running around cleaning and stuff. So what's the deal? Is body media not very accurate when it comes to measuring steps?