Matrix summation rounding error?

Dear all,

I'm having trouble with summation of matrices and difference of the two sum. Here some lines of code:

thetaMN_prev = cellfun(@nansum, theta_prev);
cost_prev = sum(thetaMN_prev,'all');

thetaMN_try = cellfun(@nansum, theta_try);
cost_try = sum(thetaMN_try,'all');

cost_prev - cost_try often returns 0, while

sum(thetaMN_try - thetaMN_prev,'all') ~= 0, but this should be the same calculation mathematically speaking.

When this happens, cost_try and cost_prev are of the order of 1e19.

Is this an error due to some rounding process in function sum(A,'all')?