选择查询返回无效的结果与日期时间字段

我有一个名为dt_cadastro的DateTime字段，其内容为15/04/2013 11:14:59，然后我执行此查询

SELECT descricao, dt_cadastro
      FROM MyTable
      WHERE dt_cadastro   >= '11/05/2013 00:00:00' AND dt_cadastro <= '11/05/2013 23:59:59';

它确实返回这一行。为什么?如果我指定的DateTime值大于我的表上的日期…有人能给我解释一下吗?我试图使查询返回我的所有行在日期的间隔，如Where dt_cadastro >= @date1 AND dt_cadastro <= @date2。但它仍然返回我行，当它应该返回我EmptySet。我用的是c# - Asp.net。我也试过使用Between，但当使用between时，它永远不会返回我任何值…
Sample

 string sql = "SELECT proj.descricao AS projeto, func.descricao AS funcionalidade, clb.clube AS cliente, ch.descricao
              FROM MyTable ch 
                   INNER JOIN projetos proj ON ch.projeto = proj.id 
                   INNER JOIN funcionalidades func ON ch.funcionalidade = func.id 
                   INNER JOIN clubes clb ON ch.clube = clb.id
              WHERE ch.dt_cadastro >= @dt_inicial AND ch.dt_cadastro <= @dt_final";
 MySqlCommand cmd = new MySqlCommand();
 cmd.CommandType = CommandType.Text;
 cmd.CommandText = sql;
 cmd.Parameters.Add(new MySqlParameter("@dt_inicial", MySqlDbType.DateTime)).Value = rel.Dt_Inicial;  
 cmd.Parameters.Add(new MySqlParameter("@dt_final", MySqlDbType.DateTime)).Value = rel.Dt_Final;