It looks shes still pretty young, so her tail probably has a bit more growing to do
Did you get her as a baby betta?
Her tail might look like this
or something a bit closer to the body
Basicaly if I remember correctly a double tail is betta who has 2 lobes verses 1 whole one. Its kind of like if you took a whole tail of a betta and split it into 2 tails that equal the same of the whole. Like dividing it into two.
If that helped at all
As a comparison
Ah thanks for taking the time to get all those pics!
Yeah, she is pretty young. I got her 2 weeks ago, she's grown quite a bit since then already!
The top part of her tail is a bit smaller than the bottom.
I wish I could get a better picture but every time I try she comes right up to my phone so I end up with a close up of her face lol