If you are a SAHM, is there a Daddy?
Girls often don't 'believe' mommas telling them something physical is going to be safe, but are more likely to believe their dad or another man who cares deeply for them. Presuming said dad doesn't push them 'past' their fear like he would push a boy.
Girls need to have their fear accepted to move through it, from the inside, and teaching a girl 'there's nothing to fear' through physical experience (from the outside) probably won't work like it would for a boy. As far as I can tell (from growing up and now having kids), a boy jumps off something high once and doesn't die, it's all fun from there ... a girl jumps off something high and doesn't die, it's still scary anyhow, for a long time/many more jumps.
Riding a bike freaked me the heck out, for months, when I learned at 6. Currently it's freaking my 4 year old daughter out--but my 6 year old son finds it frustrating, not freaky.
Driving a car about 15 mph for the first time freaked me out, too, when I was 15 ;).