Sequential: large amounts of data in order (multi-megabyte).
512KB: blocks of 512KB. So, write 512KB here, another 512KB there, etc., then for reading them back, read them back in a predictable pseudo-random order.
4KB: as above, but with the smallest logical size that matters.
512KB would be representative of largish business documents, small pictures (by today's standards), compressed scanned 1-page documents, etc..
4KB is commonly written, but less commonly read, and would be representative of configuration files, icons, bits of logs, lock files, etc..
RAID 0 works in stripes, a contiguous block of data that each drive gets, and usually 64KB (with cheap disks and RAM, these days, if you need to adjust stripe size, you really need faster drives). While a given read does not necessarily need to read a whole stripe, nor write a whole stripe, it still has to manage and update a whole stripe, logically, so there's some overhead in safely handling that.
On top of that, at QD=1, each read or write needs to complete before the next one is issued, so the latency of the drive and AHCI/RAID/SATA limit how fast it can be done. Most drives likely a lie a bit, when it comes to writing, as well, which would help explain the difference (some claim not to, but most ignore the subject, and consumer drives have a long history of lying about data actually being committed, to improve speed).
On of that, since you're never going to be limited to a QD of 1 in practice in a RAID 0 of SSDs, the QD=1 bench is really only useful as a check to make sure things are working like expected, not good as a performance comparison. But, by the same token, since you'll basically never have a QD of 32 per drive for more than a brief instant, outside of synthetic benchmarks, the QD=64 bench, while useful, isn't really telling you how your drive will perform (PCMark and Iometer are among the best all-around tools for that, IMO).
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