Wait_Time

I am trying to understand the Wait_Time column in sys.dm_exec_requests.

BOL says concerning Wait_Time: "If the request is currently blocked, this
column returns the duration in milliseconds, of the current wait."

In executing sys.dm_exec_requests, all my blocking_session_id's are zero, yet
some of the Wait_Time values are non-zero. Also some of the non-zero values
are negative.

Please explain:
1. If there are no requests being blocked (all blocking_session_id values are
zero), how come there can be an non-zero value in the Wait_Time column?

2. Since there are some non-zero values in some of the Wait_Time column, how
can one have a negative Wait_Time?