Why is there a difference between these transaction DMVs in SQL Server 2008 R2?

Question

When I execute the two queries below,

SELECT 
    session_id, transaction_id
    FROM sys.dm_tran_session_transactions;

AND

SELECT 
    session_id, request_id, at.transaction_id
    FROM sys.dm_tran_active_transactions at
        JOIN sys.dm_exec_requests r
            ON r.transaction_id = at.transaction_id;

I've read the BOL for both 1 and 2 but don't see any clear explanation as to why the difference would occur.

I get different results. The former query returns no results, but the latter returns active transactions with session and transaction ids. The request_id is 0 which, I think, means that it's the only request made by the session. Could someone help me understand why there is a difference between the two concepts I've queried above?

EDIT

I just reran the queries and now I get a result for the first DMV which has a session_id that is not actually contained in the second result set.