Hello Jaeun,
The leadfield matrix is a matrix that is dependent on the technique you use to make it (Boundary element method or Finite Elements) and is dependent on the individual head model.
It represents how possible sources effect the EEG or MEG electrodes. Saying that, for the same API and the same head model with its conductivities and solving method the leadfield matrix should be more or less the same.
It should, for one person, not change over trials, except if the electrodes are moved.
Kind regards,
Steven