For those less familiar with x86, the example instruction encodings that are interpreted differently on Intel and AMD are not base encodings, but instruction encodings modified with prefixes.

The x86 ISA includes a great number of bytes that are used as instruction prefixes, many of which are obsolete. The problem is that the effect of prefixes upon instructions has never been completely defined in any Intel or AMD documentation. The prefix effects have been documented for some instructions, but they were left unspecified for most other instructions.

This has lead to divergent implementations in the unspecified cases. Well-behaved compilers should not generate such undocumented combinations of instruction prefixes with base encodings.