Lsqr first iteration without any initial guess

I am trying to solve argmin||Ax-B|| using lsqr with a function handle and without any initial guess. I thought that lsqr would have performed the first step computing x by A'B using the vector B I passed to the function. This is the code (W_vector is a weighting vector):

   b=W_vector.*B;
   [x, flag]=lsqr(@(x1,modo)FUNC(x1,W_vector,A,modo),b,tol,maxit);

    function [result, modo]=FUNC(x1,W_vector,A,modo)
        %Computes y=A*x for modo='notransp'
        %Computes y=A'*x for modo='transp'

        switch modo
            case 'notransp'
                res=A*x1;
                R1=reshape(res,norient,dim(1)*dim(2)*dim(3));
                for co=1:norient
                    R2(:,:,:,co)=reshape(R1(co,:),dim(1),dim(2),dim(3));
                    FR(:,:,:,co)=ifftn(R2(:,:,:,co));
                    aux=FR(:,:,:,co);
                    R3(co,:)=aux(:).';
                end
                result=W_vector.*R3(:);

            case 'transp'
                R1=reshape(x1./(W_vector+eps),norient,dim(1)*dim(2)*dim(3));
                for co=1:norient
                    R2(:,:,:,co)=reshape(R1(co,:),dim(1),dim(2),dim(3));
                    FR(:,:,:,co)=fftn(R2(:,:,:,co));
                    aux=FR(:,:,:,co);
                    R3(co,:)=aux(:).';
                end
                result=A'*R3(:);
        end
    end

When I checked the result of R1=reshape(x1./(W_vector+eps),norient,dim(1)*dim(2)*dim(3)); in case 'transp' in the first iteration, I found out that x1 is not equal to b (after reshape, they are images, so I can see that look qualitatively the same but with very different values). I would like to understand how lsqr works because I am having some problems with weightings. Thanks for your help.