The agent gave you a BS answer. I have had the same thing happen just because some couple wanted to sit together. In my instance, I noticed the same thing about 15 min before boarding. I asked the gate agent and they claim they paged me (I wasn't at the gate, I was sitting in the lounge) and "were going to put $25 in my travel wallet". I had some choice, yet polite, words for the agent letting them know I wasn't very pleased and if they had intended why didn't they deposit the money before I questioned them? I wouldn't have been so pissed if they had moved me to another aisle seat but they moved me to a window in the back. They did find another aisle seat for me, but it was still frustrating.