Yes, it`s very normal after a hair transplant for one side to grow quicker than the other. It doesn`t happen to every patient but if it happens to you, then you naturally worry. It can happen where one side does not get the same growth as the other side after 12 months, This can be due to blood supply etc. If that happens, then the clinic will usually do a free touch up to add a bit more density.
Try not to worry and over scrutinize as chances are everything is fine, just one side a bit slower.