This is only a possibility of course, but maybe Martha had the first three before marriage, then married Robert later:
1841 HO107/176/92 fol.6 p.6
Newby, Cumberland
RICHARDSON
Thomas 52 Farmer
Margaret 43
Edward 20
Martha 18
John 14
Thomas 13
Mary 11
Dorothy 8
Joseph 5
all born in county
1851 HO107/2431 fol.230 p.
Newby, Cumberland
RICHARDSON
Thos, head, m, 61, Farmer of 61 Acres Dalston Cumb
Margt, wife, m, 53, farmers wife, Orton Rigg, Cumb
Margt, dau, s, 28, farmers dau, Dalewintleton
John, son, s, 24 " son "
Joseph,son, s 16 " " Orton Rigg
Dorothy, dau, s 18 " dau Heighby
Margt, granddau, 9 " g.dau Newby
Thos. " son 7 " g.son "
Henry " " 1 " " "
Martha & Robert are living together I think in 1851, will post later
Barbara
Ditto Geoff!!!