Since SoSSE version 10.1 the deadlocks are recorded using SQL Server’s extended events functionality. This is the preferred method for detecting/reporting deadlocks in SQL Server.
Extended events will report more deadlocks than the old technique. In particular, “Intra-query parallel thread deadlocks” are now reported. These occur where a single query is split up into multiple threads to be executed, and these threads deadlock each-other.
(There are blog articles on this and what to do about them outlined Here)