Why does mattest output different p-values when using bootstrapping?

If I use the following code

a=[1:20,1:20]
b=[2:21,2:21]
pvalues1=mattest(a,b,'Bootstrap',1000)
pvalues2=mattest(a,b,'Bootstrap',1000)

pvalues1 does not equal pvalues2, however pvalues1(1)==pvalues1(2) or pvalues2(1)==pvalues2(2)

Why does this behaviour occur?

[Sreeja Banerjee]

Hi Andy,

Which release of MATLAB are you using? Also what values do you get for pvalues1 and pvalues2? I think the data is ill-formed because I am not able to reproduce the same behavior.