x86 CMP Instruction Difference

It doesn't matter which opcode you use if you compare two registers. The only difference is when comparing a register with a memory operand, as the opcode used determines which will be subtracted from which.

As for why this exists: The x86 instruction format uses the ModR/M byte to denote either a memory address or a register. Each instruction can only have one ModR/M value, which means it can only access one memory address (not including special instructions like MOVSB). So this means that there can't be a general cmp r/m32, r/m32 instruction, and we need two different opcodes: cmp r/m32, r32 and cmp r32, r/m32. As a side effect, this creates some redundancy when comparing two registers.

It's redundancy of x86. There are much more many cases like this. A compiler/assembler is free to use any of the valid opcodes

Some assembler allows you to choose which opcode to emit. For example on GAS you can attach ".s" to use the other instruction encoding

10 de   adcb   %bl,%dh
12 f3   adcb.s %bl,%dh