StateSample and State vector with different dimensions

I couldn’t get why it exist StateSample (24x1) and state vector (23x1)? why they’re different? and when one of them is used and the other is not? since I find that the state of the EKF is a StateSample type vector, but the covariance matrix is (23x23)?
I’m a beginner but based on what I know normally the covariance matrix needs to be 24x24?