why the electronegativity values of silicon, germanium and tin has the value "1.8" whilst it should keep decreasing from top to botom in a group

Electronegativity : 

The electronegativity for group 14 decreases from C to Si its value become constant to 1.8 from Si onwards.

 ...{C  Si  Ge  Sn  Pb}...

Pauling Electronegativity  2.5  1.8  1.8  1.7  1.6 value.

Reason :

The electronegativity from silicon or shows a very smaller decreases as compare to other elemnts of other group. This is due to poor screening effects of d¹⁰ electrons in Ge atom and f ¹⁴ electron in Pb  which balance out the increases in the size of atom and increase the nuclear charge more then decreases in  size. Hence electronegativity increases with less amount as compare to other element.