Total speculation, but the "5" could well be because the BIOS displays whatever the CPU had in its DH (the upper half of DX) register after reset. I remember a

Total speculation, but the "5" could well be because the BIOS displays whatever the CPU had in its DH (the upper half of DX) register after reset. I remember a few tools that were made before the Pentium was a thing, that also reported "586" when you tried them out on one. (If I remember correctly, the BIOS also has/had a BIOS function to relay that value to a program.) From the 80386 programmer's reference manual: "DX holds a component identifier and revision number after RESET as Figure 10-1 illustrates. DH contains 3, which indicates an 80386 component."