You’re meant to wonder that - it’s part of an intentional narrative. How is it possible? Because police don’t arrest everyone they detain. For one documented example of this, look to the Cop City protests in Atlanta:

(Sorry about the paywall, so quoting below):

There’s a certain irony, then, that in statements on Sunday’s arrests, Atlanta police officials have made a point of blaming “outside agitators” for taking up militant action. Out of 44 people originally detained in Sunday’s forest raid, the 11 people released without charge all had Atlanta addresses. Twenty-one of the 23 activists charged with domestic terrorism are from out of state.