You know how you have a hand your prefer to write with? Brush your teeth with, scrabble eggs with amongst many other things? You’ve spent a lifetime choosing that hand for most tasks over your other side. The same thing has happened with your legs. Your whole life, you’ve preferred to stand on one leg more than the other, taken first steps with it, tested the waters on uncertain ground, caught yourself when you tripped and used it to plant into the ground to jump or kick.
This, unfortunately, results in having one leg that you feel rock solid stable on, and leaves us with a slightly less controlled, wobbly side. For me, my left leg is as stable as a tree trunk, my right side, not so much. In fact, I always make sure to demonstrate one leg SLDL’s on my left leg because my balance is so poor on my right!
Now, this is a simplified explanation of this. There are a lot of other things happening. One side probably has more range of motion than the other and doesn’t get as tight or as sore. Other factors can contribute to this, but a great way to make some real world change is awareness - try to stand on the less preferred side more!
I’m not sure your SLDL’s will ever even out, I’m sorry there isn’t a magical balance drill that will solve that. Through your awareness though, you can start to let that other side get some reps in and stand on it and try to use it just as much. When you get frustrated, remember how difficult it would be to write, brush your teeth and throw a ball with your non-dominant hand.