The study addressed this. There are sexist gender roles at play even among single parents, potentially. (To wit, you didn’t lament the rise of “single fatherhood.”)
From the study,
> These results suggest that single men are able to take care of their dependents by working more overtime, possibly to pay for child support or to finance other forms of child care. Single women, on the other hand, appear to be making the decision to do the caretaking themselves rather than to caretake through additional earnings. It is, of course, possible that for women this situation is not as much a personal preference as it is a constraint.