Why does the time I tell about the graph vary?

According to me, I am measuring the time since the data arrives to the cell (1,50) -> v2(1,50) but, matlab is giving me different values when executing my code, why does this happen? I want to measure the time from when you start graphing to the value of 90 (that is, cell (1,50)) [I put a break to stop graphing at the point where I want to finish counting time, I am sending the data from arduino to matlab]

function arduinomatlab


v1=zeros(1,1000);
v2=zeros(1,1000);
delete(instrfind({'Port'},{'COM3'}));
s = serial('COM3','BaudRate',9600,'Terminator','CR/LF');
warning('off','MATLAB:serial:fscanf:unsuccessfulRead');

fopen(s);

muestras=1;

figure('Name','Captura');
xlh = xlabel ( 'Grados' );
ylim([-1 1]);
xlim([0 369]);
ax = gca;
ax.XAxisLocation = 'origin'; 
xticks([30,60,90,120,150,180,210,240,270,300,330,360])
ylabel('Y')
title('Onda Sinouidal')

grid on
hold on


while 1



try
valor=fscanf(s, '%f,%f')';
v2(muestras)=(valor(2));
tic
v1(muestras)=(valor(1));
if v2(1,50)
tiempo=toc;
disp(tiempo);
break
end
plot(v2(1:muestras),v1(1:muestras),'-k')
drawnow
muestras=muestras+1;
catch
break
end
end   
fclose(s);
delete(s);
clear s;

end