Another solution/hypothesis is that Matthew remarried and liked the name Mary!
There is an really remote possibility: Free BMD gives the marriage [among 3 other choices] of Matthew Grose and Mary Treggoning [a good Cornish name!] at Holywell in Sep Q 1839. She may have had to return to Cornwall or elsewhere in 1841. She would probably not list Matthew's children as stepchildren in subsequent censuses.
GRO Sep Q 1839 Holywell 27, 259 would disprove [maybe] this wild surmise!
JohnL