Pl. find the extract from BOL:

"In SQL Server 2012, statistics are not created by scanning all the rows in the table when a partitioned index is created or rebuilt. Instead, the query optimizer uses the default sampling algorithm to generate statistics. To obtain statistics on partitioned indexes by scanning all the rows in the table, use CREATE STATISTICS or UPDATE STATISTICS with the FULLSCAN clause."

Keep in mind that update_statistics and Create_statistics are both different.