Beryllium differs from other alkaline earth metals because of

"Be" in the 2nd group differed from the rest of the elements of the same group. This is said to be anomalous  behaviour  of "Be"
Reasons:
1) Due to its small size and high EN
2) Its high polarizing power[P.P]
 

 polarizing power[P.P] =$\frac{\mathrm{Ionic} \mathrm{charge} }{{\left(\mathrm{Ionic} \mathrm{radii}\right)}^2}$

Because of its high polarizing power it mainly forms covalent compounds
Eg :- BeCl₂