Rear Frame Width Question

[TripleNickel]

I've seen the thread with frame dimensions, but this has me puzzled.

I recently purchased a 1972 C-30 with factory dual wheel option.

For comparison, I measured 'outside to outside' dimensions of the rear leaf springs on the C-30 to compare to the same dimension on a 1972 GMC LWB 3/4 ton which also has an original leaf spring setup.

I was surprised to find the width at the springs was 2" narrower on the C-30.

It was my understanding that the frames are equal width (at least to the back of the cab) on the C-10s, 20s, and 30s.

Do the frame widths behind the cab vary, or is this 2" difference something involving the spring mounts?

Thanks in advance for any insight that can be provided.