From **broken link removed**,
it would seem that the "S" in "SP" only implies that it's the skinny DIP, used solely to differentiate the packaging from the non-skinny. :shock:
Anyway, do you think the infamous 16F877 is 300mil?
As to why they have diff range of products for low voltage operation, I'm not too sure, but there could be a variety of reasons.
1. Market segmentation
2. Newer design
3. Just different labelling after going through performance evaluations. (No 2 chips are the same after all!)