Why can a full helmet of MEG sensors still not say for certain where a signal came from? · Grey Matter

The answer is not unique.

Why can a full helmet of MEG sensors still not say for certain where a signal came from?

The answer is not unique. Helmholtz proved in 1853 that the currents inside a conductor cannot be recovered from the fields outside it, since some current patterns produce no external field and can be added to any solution unseen. A few hundred sensors facing many thousands of possible source locations leave infinitely many current maps that fit the data equally well. MEG source maps therefore come from adding assumptions (a few point sources, the smallest total current, sources confined to the cortical sheet of the subject's MRI), and different assumptions can place the same signal differently, especially for deep sources.