Help me with this puzzle

I have located my gt grandparents Smith & Elizabeth Bradbury's grave.Buried with them is a 23 month old babe (not theirs)Babes name on grave is Thelma Elizabeth PRETTY.I checked BDM and found the birth & death reg.Parents names of babe were Martha Jane BRADBURY and James YATES.So far I can find no Martha Jane in my family tree who could have been the mother but Martha was the name of Smiths mother so is a family name.Smith & Elizabeth did have a daughter called Margaret Jane so she could have been babes mother.My querie is why would the babe have the surname Pretty when parents surnames were Bradbury & Yates.Can anyone offer any suggestions..

Thanks
Jenny Fairchild