Both v$librarycache and v$rowcache collects statistics regarding the usage of the shared pool.
The shared pool holds both the data dictionary cache and the fully parsed or compiled representations of PL/SQL blocks and SQL statements. PL/SQL blocks include procedures, functions, packages, triggers, and any anonymous PL/SQL blocks submitted by client programs.
Therfore,the collected statistics is on the memory usage and not on the data usage.
The interviewer says v$rowcache is on data usage......