There are some types of log ins that Safari never asked me for either. There's something similar about them, but I don't know squat about code! Haha. It never asked for Yahoo! mail or the log in for my telephone bill. But then on other things, including secure (https) log ins it did.
Someone else might know what the difference is between the various log ins, but I think it's normal that Safari doesn't ask for all of them.