why etc1 in cocos2dx is 24 bit per pix?

I see some articles says that the etc1 has 4bit for every pix.

But in the the cocos2dx(I use the 3.0beta2), I got the dump info shows that etc1 has 24bit for every pix.

#ifdef GL_ETC1_RGB8_OES
PixelFormatInfoMapValue(Texture2D::PixelFormat::ETC, Texture2D::PixelFormatInfo(GL_ETC1_RGB8_OES, 0xFFFFFFFF, 0xFFFFFFFF, 24, true, false)),
#endif

Could someone explain this to me?

did someone know this issue?

when loaded into memory, they are all the same, etc1 is 4bit per pixel when stored on a file