I used to take lessons at a riding school and this always used to happen to me. Once I got comfortable and started getting used to a horse, my instructor would swap me to a different one. It used to really annoy me until I realised that she was doing it to help me. Each different horse would help with some aspect. For example, I have always had trouble wih being a stiff rider. Once I started relaxing more on a relatively smooth horse, I would get swapped to the bounciest horse on the yard, so I could learn to sit even more so.
If you're uncomfortable with keeping swapping then tell your instructor, she'll try not to swap you so much if you're not comfortable. After all, you are the one paying for lessons!
But sometimes it is nice to ride something different, even though you get very close to one horse, different horses will always have something new they can teach you.