The chart doesn't show the types of RAM tested, so it is not very useful. It also appears to be testing different RAM types:
-- 512MB is DDR RAM or earlier;
-- 1024-2048 could be DDR2,
-- 1024 and up could be DDR3
Use google to find RAM JEDEC standards and read those, it will give a lot of insight on speed and latencies, as well as the differences between DDR, DDR2, and DDR3 RAM.