The laws of physics don't change enough for that. Counting seasonal layers, confirming flow rates, etc., are valid measurement/analysis techniques.
My point was how do they know what is NORMAL, if they don’t have the information about the previous ice sheet, and how long it stayed attached, before dropping off. The information from the layers, etc, is all well and good, but unless they’ve measure a different ‘drop’, they wouldn’t know what is the typical time frame for its occurrence.