Something strange about the storage capacity

Hello, first I apologize for my English, it is a strange thing with the storage capacity of my phone, as the capacity of photos is now 150 gigabytes, and my phone’s capacity is 128 gigabytes. Isn’t it supposed that after the capacity of photos reaches 120 gigabytes, my phone will stop storing more than the original capacity?

My phone { iphone 13 128GB}