I have an ASUS motherboard that uses AMI BIOS version 0242.

When I try to clear the MSDM table with AMI's own AfuWinX64.exe, it is unable to modify the flash.
But when I use ASUS's SlpBuilder.exe, it is able to modify the MSDM table.

Why can ASUS's tool modify the AMI BIOS, but AMI's own tool cannot?

Aren't the BIOS tables standardized? Like the MSDM table can be found in a specific memory location, and the data that it holds is standardized?

The so called "Standartized" AMI tools r service tools not ment for personal end user but to industry standards and all the sold resources available involved, this is not to u to understand as a simple consumer.
So ur surprise is really no surprise at all...wot did u expect?
Are u a IT/Bios engineering student? If u were u wouldn't be doing this kind of remarks.
The OEM has his own ways to release his contents according the needs, simple as that.