Error bars are all the same size?

Here is my code:

x = [8,9,10,12,14,16,18,20,25,30,40,50];
y = [10.7,8.9,7.3,5.5,4.3,3.4,2.7,2.3,1.6,1.1,0.7,0.5];
x = 1./(x.^2);
[c,s] = polyfit(x,y,1);
[f,delta] = polyval(c,x,s);
plot(x,y)
errorbar(x,f,delta)

Why does the plot result in the error bars all being the same size because experimentally they shouldn't be. What am I doing wrong?

[Azzi Abdelmalek]

What experiment are you talking about?

[Anna]

The experiment measured distance and radiation level (Stefan-Boltzmann law).

Answer 1

[Azzi Abdelmalek]

If you look at delta, you can see that the values are not equal

0.2935  0.2719  0.2608  0.2526  0.2515  0.2522  0.2535 0.2547  0.2571 0.2587 0.2605 0.2614

The values are slightly different, visually, you can't see the difference in the figure