Thank you for your responses but never mind this question. I feel sort of ridiculous about this, but my code for calculating the mean matrix was wrong. The ans that resulted from MyCellArray{:} was simply the last matrix, and x = MyCellArray{:} results in the first matrix.
For those of you who wish to calculate the mean of each matrix element across the 32 cells, assuming the size of each cell is the same (600x1 in this case) without using loops, I figured out the following code:
meanArray = arrayfun(@(y) mean(cellfun(@(x) x(y), MyCellArray)), [1:600])'