It is for the line, purpose and architechture
i will use intel as an example:
p4 processors=Willamette
p4 dual core, 65nm core2duo=conroe
65nm core2extreme=conroe XE
The lower clocked core2duo processors=Allendale
mobile versions of the 65nm core2duo processors=Merom
Quadcore 65nm=Kentsfield
Quad core 45nm=Yorkfield
Core2Duo 45nm=Wolfdale
i7 processors=nehalem
Each line has a codename, the dual cores have different to the quad and single p4, the mobile have different to the desktops.
But also the architecture making the i7 more efficient and powerful than the core2quad gives it a different name, even though they are both quad core processors
Also, the extreme lines all have XE on the end, so conroe XE, Yorkfield XE etc