Because actually the 64 bit systems are in reality what's called an x86-64 architecture.
From Wikipediea:
Quote:
The term x86-64 is the original naming of a 64-bit extension to the x86 instruction set specified by Advanced Micro Devices (AMD) and implemented by AMD, Intel, VIA, and others.
It extends the virtual and physical address spaces, doubles the width of the integer registers from 32 to 64 bits, increases the number of integer registers, and provides other enhancements.
It is fully backwards compatible with 32-bit code without any performance loss.
The generic term x86-64 is sometimes shortened to x64 as another vendor-neutral term for x86-64 processors from any company.
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I hope this helps explain it.