I have looked at another version of the Marriage Index and for Qtr Jan 1909 (Vol 11A Page 929)in Merthyr Tydfil there are four names:

Annie Roberts
Thomas Henry Davies
Levi John Rees
Keturah Evans

Assuming that you have the right family in 1911 I am at a loss to know why the name variation, other than perhaps simple human aberation - Thomas having a bad day!