Many dogs are bred to have certain dispositions, and often times it has to do with how they were raised (i.e. what behaviors were encouraged and which were discouraged). You must also consider that she is a female dog, and that dogs have a determined dominance system. If she was raised as being a non-dominant dog, she will naturally submit to other dogs, especially those that are aggressive towards her, and particularly males.
You must also consider that she was spayed, which is often used, not only as a form of indefinite birth control, but as a way to calm aggressive, energetic, and incorrigible dispositions. If you consider all of these things, your lovely and friendly dog may be the way she is from a combination of some or all of these things, leading her to be severely non-aggressive.
There is absolutely nothing wrong with this either, unless you had intended to use her for a guard dog or police dog which requires some aggression. However, it doesn't seem like this is the case. So, I do not think you need to worry about her disposition.
What you need to worry about, is your irresponsibility in handling her safety. Knowing how she is and how she interacts with other dogs, you are putting her in danger by allowing other strange dogs to interact with her and potentially harm her. Your invisible fence may keep her in, but it obviously isn't keeping other dogs out. You have a responsibility to keep her safe, and you are not performing your duty. I suggest you build a real fence, and only let her interact with dogs you know.
Dogs that are aggressive towards other dogs should not be allowed to interact with other dogs, it's as simple as that. Especially ones that you know will not or cannot fight back. It saddens me that you would allow your dog to interact with other potentially dangerous dogs. Please do your research and educate yourself before becoming a pet owner if you are unsure about these things.
