Confused

[JBee]

Why the differences in cM's in my son's results.

My half sister has 787cM - his half aunt.
Her son has 412cM - his half 1st cousin
which I understand

So why does my half brother's daughter have 706cM compared to her full 1st cousin's 412cM.

.................................................. .................................

Comparing with a distant relatives DNA results

My half sister has 214cM and listed as a close relative -2-3rd cousin

Half niece has 130cM - 2-3rd cousin - extended relative
Half nephew has 44cM - 4-6th cousin - extended relative
My son has 30cM - 4 -6th cousin - distant relative

i would have expected the three 1st cousins to have about the same results as they share the same common anncestor.

This DNA lark does my head in