Yup.
We had this with one of ours. Apparently it was there in utero but it came out later.
Had to do with how we had him sitting, and how he slept.
We were always putting him to sleep one way in his crib. I also fed him a certain way. I kind of naturally put him how he looked most comfortable.
So it worsened to the point where it was then 'noticeable'.
PT helped but mostly, it was strengthening his neck muscles the other way - where they were weak. So I had to always position his chair the opposite way so he'd have to turn his head to look for us. I had to flip his position in the crib (so head where his feet had been) and that sort of thing.
You can PM me for more ideas etc. if you need them but the PT will probably have all that info for you.
* I should add, ours was detected much younger than 11 months. I'm guessing probably more 4 months. He wasn't sitting 'up' completely by himself. So he was resting and looking one way that he favored and one side just got weaker.