Possibly two different mothers.
Maria Roberts and S amuel Davies are on the FMD marriage index Apr-May-Jun 1877 Wrexham
Strangely, in 1851 there is a family whose ages do not match but otherwise fit quite well:
87 Anne St Birkenhead HO107/2175
Thomas Roberts head mar 30 labourer b Denbighshire Llandeg---
Ruth Roberts 40 b Denbighshire Ll---
Mary daur 2 b Denbighshire
Robert 9 mos b Birkenhead.