Frame Buffer memory usage

Just wondering if I'm missing something here...

I'm using v.11 of the Frame Buffer. It is configured as 2,000 x 2,000 max image size, 8 bits per pixel and 2 planes in parallel. I am using triple buffering..

This should be 2,000 x 2,000 x 2 Bytes X 3 frames ~ 24Mbytes of storage.

However, the GUI is reporting 46.875 Mbytes required?

Does anyone have an explanation for this?