Hello, the difference you noticed has to do with the daylight saving time exception for the Navajo Nation, as is written in timeanddate.com at https://www.timeanddate.com/time/us/arizona-no-dst.html
Most of the US state of Arizona does not use Daylight Saving Time (DST). The exception is the Navajo Nation.
When you were born, daylight saving time was "on" in your area, even though in other parts of Arizona it was "off". 12andus applies this exception. To reflect this, the timezone for your birth town in your profile is set as America/Denver (where they apply daylight saving time), instead of America/Phoenix, where they don't.
Just as a test, if you edit your birth data as if you were born in Phoenix, you will get Libra Ascendant.
However, things are much more complicated than that. As we can read in timeanddate.com,
A part of the Hopi Nation, which lies within the Navajo Nation, follows Arizona's no-DST rule. To confuse matters more, there is also an even smaller Navajo Nation territory within the Hopi Nation within the Navajo Nation. In addition to this, there is another Hopi area adjacent to the main Hopi Nation territory.
As a result, if driving the correct route from the Arizona state border through both Navajo and Hopi areas to the other side one can end up changing one's clock 7 times! For example: Tuba City (Navajo) and Moenkopi (Hopi) are only a couple of miles apart, but they have a 1-hour time difference during the summer. Jeddito (Navajo), in the middle of Hopi Nation territory, is 1 hour ahead of the surrounding areas during summer.
Things are very complicated in that area regarding daylight saving time. I don't know whether the place you were born falls inside one of those exceptions inside the exception mentioned above. In other words, if you were born during daylight saving time then 12andus' calculations are correct because we applied the Navajo exception. But if you are in the exception inside the exception, then it "might" be wrong according to the nature of the exception.
Probably you can find out better than me and better than timeanddate.com whether the town you were born falls inside any of the subtle exceptions. In case you discover that there wasn't daylight saving time in your specific town, please tell me.