Take the SN54AHC125/SN74AHC125 tri-state quad buffer, for example. Data sheet is here: http://www.ti.com/lit/ds/symlink/sn74ahc125.pdf
Refer to page 7, and you will see this (snippet):
SN74AHC125DGVR, SN74AHC125DGVRE4, and SN74AHC125DGVRG4 (the TVSOP entries; middle of image) have the same properties according to that chart. What's even weirder is that chips with the same properties, from the same manufacturer, might cost different! I looked through the datasheet to see if it had anything to do with packaging, but I still couldn't find anything.
So why do multiple part numbers exist that reference the same part/configuration?
The reason I actually asked this question is that I'm interested in buying a small-ish quantity of said buffer chip. I want this one, but I also see a seemingly identical chip (with the addition of G4 at the end of the part number) for a few cents cheaper. I thought there must be a reason it's cheaper, but I (obviously) couldn't figure out why.