Why does my rat keep biting me?

I have had my rats for about 5 months now and one of them is comfortable with me it runs out of the cage and into my hand for treats. But the other one is always hiding it will come grab treats from me and then go hide again. But the 2nd rat will randomly bite me! It is so frustrating. I always have the sunflower seeds held in a way that they are pointed out of my hand towards the rat... so that way there is no confusion as to what is the treat and what is my hand. So this is what happens. I will open the cage and offer her a treat... she will come up, smell the treat, then bite me. I have bled 2 times now, and escaped the 3rd time with just a nick on my finger. I don't know what to do. I treat them both the same yet one rat is nice and the other is a biter. I can not keep her if this behavior continues. Both rats were purchased at the same time from the same place. They were raised by the same breeder and lived in the same cage in the shop and at my home.