It is understandable and it is even possible. People you knew, lets say in high school, 30 years ago you may not remember today. I, for instance, do not remember a certain student in high school 30 years ago and yet she claims we were friends back then. I do not remember this. I do not even remember her. But, yes, it's understandable. You could also ask, " If I may ask, can you remind me how we know each other?" They will respond with how they know you and maybe even refresh your memory.