A razão da nomenclatura AMD é porque foi a própria que criou e introduziu a especificação 64-bit no segmento computer processor microarchitecture.
The original specification, created by AMD and released in 2000, has been implemented by AMD, Intel and VIA. Various names are used for the instruction set; prior to the launch, x86-64 and x86_64 were used, while upon the release AMD named it AMD64. Intel initially used the names IA-32e and EM64T before finally settling on "Intel 64" for its implementation. Some in the industry, including Apple, use x86-64 and x86_64, while others, notably Sun Microsystems (now Oracle Corporation) and Microsoft, use x64. The BSD family of OSs and
several Linux distributions use AMD64, and so does Microsoft Windows internally.
The AMD K8 processor was the first to implement the architecture; this was the first significant addition to the x86 architecture designed by a company other than Intel. Intel was forced to follow suit and introduced a modified NetBurst family which was fully software-compatible with AMD's design and specification.
x86-64
https://en.wikipedia.org/wiki/X86-64