Hello Narad
It is true that the startup-config is saved within NVRAM. When a Cisco device boots up, it will look in the NVRAM and it will search for the file named startup-config and it will load it into RAM as the running-config. However, it is possible, and it is common practice, to have several configurations saved within a device. These are not active but are saved as files of different names. These files can be saved in NVRAM or flash. They are simply being stored.
They can easily be enabled by simply copying them to NVRAM and renaming them to startup-config.
I hope this has been helpful!
Laz