I had issues understanding it myself, but then again it makes sense.
The ?? operator returns the "left-hand" operand if it is not null.

$b = 16;

$b ?? 7

Since the "left-hand" operand is not null 16 is returned, then after this,
we then evaluate the following

$a ?? 16
and as a result 16 is returned because the left-hand operand is null..