Calculating Response of a system with lsim versus convolution

A square wave is applied to two high-pass and low-pass filters in the following codes. Could you explain why lsim shows different response than convolution for the high-pass and almost the same response for the low-pass filter? What are missing in these codes to calculate the response correctly?

syms s

tfunc1=(5e-25*s^2+1e-08*s)/(1e-24*s^2+1e-08*s+1); %high-pass filter

tfunc2=(5e-17*s+1)/(5e-25*s^2+1e-08*s+1); %low-pass filter

tfunc=tfunc2; %choose tfunc1 for high-pass filter and tfunc2 for low-pass filter

[symNum,symDen]=numden(tfunc);

TFnum=sym2poly(symNum);

TFden=sym2poly(symDen);

sys=tf(TFnum,TFden);

[yy,tt]=impulse(sys);

[xt,time]=gensig('square',tt(end),4*tt(end),tt(2)-tt(1));

out=(tt(2)-tt(1))*conv(yy,xt,'full');

lsimplot(sys,xt,time)

hold on

plot(time,out(1:length(time)),'r')