What is it about dads?

When it comes to dicipline, what is it that dads have that moms don't? It seems that kids are always afraid to disobey dad but they couldn't care less what mom says or does. Lately I have been frustrated because I can't get Lily to do anything I say. I try to sound as stern as possible but she just smiles and continues on with what she is doing like it's some kind of game. I need to take lessons from Jacob. All he has to do is say it once and whatever he is asking of her is done in about two seconds.  I just can't figure out what I am doing wrong. I always follow through with the punishment I say I will do if the behavior continues and she knows I will do it but for some reason or another she just doesn't care. In fact the other day she kept hitting me and I told her if she did it again I would put her in time out. She walked right over to me with a little smirk on her face and hit me. Then she walked to her room and held her arms out waiting for me to pick her up and put her in her crib where she has time outs. Whenever Jacob can hear a little argument I am having with her, he comes to help and as soon as she hears him coming she cries because she knows she is doing something wrong and that now she has to answer to dad. I just can't figure it out because Jacob is far more patient and loving in his ways of dealing with these situations than I am but for some reason he is more intimidating. Anyway as you can see I am just having a rough day. This pregnancy has taken a turn for the worst so I am never feeling well and I can't figure out why my sweet , loving, snuggly little baby girl won't stop hitting me. Ugh!