Z-score for human-microarray and mouse ish-data

Hi Jeremy,
I have looked on the forum a bit and read through the documentation and haven’t found an answer to this question, so I assume the answer is very obvious. What do z score values of exactly 0 in the IVY GAP downloadable data mean? Do they either mean that there is a missing data point or that the z score was actually exactly zero?
Thank you,
Austin Kunch