Because select * from #t1 where id in(@p2) is completely and totally equivalent to select * from #t1 where id = @p2. Each item in an IN (variable, parameter or constant) is considered to be a single value
To specify variables in an IN, it would be like this: select * from #t1 where id in(@p1, @p2, @p3, @p4)
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability