I looked up the story behind the "Bluets and Granola Bars" easter egg. Apparently, the System Error Handler, part of the boot ROM, used to display a popup that

I looked up the story behind the "Bluets and Granola Bars" easter egg. Apparently, the System Error Handler, part of the boot ROM, used to display a popup that said "Welcome to Macintosh" at startup, before the operating system loaded. However, in System 7.6, this was replaced by a new splash screen with the Mac OS logo, and the old "Welcome to Macintosh" message in the system resource file was replaced with the Granola message (probably under the assumption that it would never be shown). However, if the operating system fails to suppress the boot ROM's popup for whatever reason, the Granola message shows through.