A notable aspect of Sybill Trelawney’s character is that she has no recollection of the prophecies she has made. Is it possible that she has made other prophecies which no one happened to be around to record? She seems to spend most of her time alone in her tower.
I guess we will never know, if it happened when she was chilling alone. But the two prophecies we know she made were made when there was just one other person in the room. Maybe the gift she has senses this and the prophecy only comes out when there is someone to hear it alone (or alone as far as she can see)?
It is not a big dataset, but it is a pattern.