sys.dm_db_partition_stats - incorrect row count for InMemory tables
Details about In-Memory Tables cannot always be seen using the "normal" dynamic management views, or DMVs, such as sys.dm_db_partition_stats.  

Instead, you'll need to use the In-Memory views detailed on MSDN.

For instance, this page talks about how to calculate row and table size for In-Memory tables.

MSSQLTips has a decent starting list of In-Memory related DMVs.