This machine have 4GB, and ZFS have very agressive caching, probably more than other file systems. I do no think the random write test was done with such big files, as it will ages to complete in really random write way. Having for example few-hour Postgresql benchmark on 40 GB database would be probably better indicator.

But result of 49 MB/s is still slightly below hardware possibility of this disk as I see (about 55MB/s). Having 100MB/s would be more obviously wrong (indicating that some blocks are written multiple times probably).

Other possibility is that compression was enabled, but it would also boost continious write results. So can be excluded from possible causes.