As I read the stat it measures the number of fathers who find out they are not the biological father---- not who is living with whom--- which are 2 different measures.  And even if you are correct in your interpretation,  maybe its because many single mothers  remain single or  later form lesbian relationships, which could explain the 12 per cent gap? Do not know... just speculating. 
As for the lesbian mothers, 100% of the partners know they are not the biological father or mother... far higher than the 30% figure for men... So what?   What is the point of this statistic?  Is there some old school moral condemnation of women lurking behind the use of this stat, as evidence of their "deceit"? 

 To which I would ask, what  percent of men cheat on their partners?  I bet its higher than 30%.  This game of trying to assert  a gender moral high ground on the basis of stats is juvenile.