It isn't possible for crash detection to differentiate between "phone falls off bike" and "bike crashed" because everything that happens to the phone could be identical in both scenarios.
If the phone is loose in a pocket (or even in a backpack) when the bike crashes, it can easily hit the ground, bounce, and then come to a stop... just like it does if it falls out off the bike in any non-crash scenario.
It isn't possible for crash detection to differentiate between "phone falls off bike" and "bike crashed" because everything that happens to the phone could be identical in both scenarios.
If the phone is loose in a pocket (or even in a backpack) when the bike crashes, it can easily hit the ground, bounce, and then come to a stop... just like it does if it falls out off the bike in any non-crash scenario.