Isso é explicado mais claramente em info uname
:
'-m'
'--machine'
Print the machine hardware name (sometimes called the hardware
class or hardware type).
'-p'
'--processor'
Print the processor type (sometimes called the instruction set
architecture or ISA). Print 'unknown' if the kernel does not make
this information easily available, as is the case with Linux
kernels.
O nome do hardware é a arquitetura , enquanto o tipo de processador é o nome do instruction set usado. Para citar o wikipedia:
Instruction set architecture is distinguished from the microarchitecture, which is the set of processor design techniques used to implement the instruction set. Computers with different microarchitectures can share a common instruction set. For example, the Intel Pentium and the AMD Athlon implement nearly identical versions of the x86 instruction set, but have radically different internal designs.