The issue is usually that you only get the sequential number (which is the highest), and you will realistically never get that speed since you would need to only read/write a single large file at a time (like an NVR is one of the few applications where that's true). To complicate it further the number reported is with DRAM cache unsaturated, if you actually sustain that write speed for a longer period it will also slow down to less than 1/3 iirc.