ROM on android phones isn't really "read only"... right?
It seems like you've been misinformed. ROM means Read-Only Memory and is used in Android as such.
The ROM is the part of your system that is: able to run on its own, write protected and thus not changeable.
Many/All manufacturers implement a way to bypass Linux' restrictions/file system permissions to be able to update their ROM ie. write to /system.
The normal user is not able to write to that /system partition, which is independent form the /data partition, where all your apps and data is stored (/data is not needed to run Android).
The only thing those two partitions share is (not always) the same hardware/memory chip.


  The contents stored on /system contain all software needed to run Android on that specific device including: drivers, configurations, scripts, applications, frameworks etc.
  They are read-only for the user and only writable after remounting as a system user or superuser/root.
  Therefore ROM can be considered a valid and correct term for the Android system's /system partition/ROM partition.