I don't think you're doing anything wrong.
Are you right-handed? If so, you're probably naturally stronger on the right than the left since the right gets used much more often in everyday life. So the muscle probably feels stronger and firmer.
Also, people lose weight in strange and uneven ways. Some people say they lose in one boob before the other, so I imagine it might be true for arms also. And if there's less fat in one arm, you'll see and feel the muscle better.
I actually had a size difference between my right and left upper arms when I was losing -- which I assumed was due to me being right-handed. It was weird to try on clothes and have one sleeve fit and the other be too tight. It evened out in the end but it took a while.
We're all stronger on one side than the other -- arms, legs, shoulders, calves. When you do your arm exercises, do you go to failure (until you can't do one more rep)? The way to even up arm strength is to take the weaker side to failure and then match the same number of reps with the stronger arm. Eventually the strength differences equalize, and hopefully the size and shape of the muscle also.