After seeing this myself several times with T1, I did finally ask her about it. She seems strict on her time boundaries, yet I'd see her let some people stay to the top of the hour (she also holds 50 min sessions). She told me that there are some clients she only sees once a month, for "maintenance" sessions so to speak, and those clients she sees for a full hour, since they don't come in weekly. She also sees couples for a full hour, but that one I knew.