In short, the difference is that you can flash it from the native GUI whereas this does not work with the official images.
In long, there are three more headers and some size constraints on the image, that seem to be strictly checked by the native firmware:
The fragment sizes need to be exactly the flash partition sizes, as pointed out in one additional upfront tplink1.1 header (easy). But there is two more headers, 36 or 40 bytes with some length fields and an md5 checksum, plus another 20 bytes header (length+checksum), where I didn't manage to find the seed or the extent of checksummed data yet for these two; ie I'm using a tool from the tp-link sdk for creating these.