Calculating Overall Relative Bias

I am in some trouble to understand how is to calculate the overall relative bias. In this link, there are results of overall relative bias in "Parameter estimates" sub-section under "Results" section. There they mentioned that :

the overall biases for the fixed effect parameters \(\gamma_{00}\), \(\gamma_{01}\), \(\gamma_{10}\), and \(\gamma_{11}\) were 0.6%, 2.6%, 1.4%, and 3.7% respectively.
Could you please explain me how did they calculate it? I had thought that if I calculate the mean of the column of \(\gamma_{00}\) in table 1, that is, \((8.77+4.75+3.94+\ldots-0.06-0.14)/27\) , I would get overall mean of \(\gamma_{00}\). But it doesn't match with the result.

Also, they wrote that

When the size of the group was increased to 30 with 30 groups, the bias was reduced to less than 6%.
I am not understanding how have they calculated that the reduced bias is 6% in this case?