Memtest (depending on the version) can only test the first 4GB of physical RAM, even if the user initiates it in single user mode. ATOMIC, on the other hand, can test well beyond the 4GB limit of Memtest. While Memtest86 can test past the 4GB limit, it, like ATOMIC, can only test free available memory.
Also, some of the tests that Memtest uses are very redundant with other tests within the Memtest/Memtest86 application. So it gives the appearance that there are many memory testing algorithms, but most of them are subsets of another test. Each test algorithm in ATOMIC is unique to itself.