Anomaly.
It happens. That's why it's good there's more than one review to look at.
Try to think: you can get unusually low results for a card. But you cannot get unusually high results. You cannot score a 3DMark record by accident can you? Has anyone accidentally scored a too high score in benchmarking thread on this forum? And especially not in a broad range of titles.
For example again and again there happen situations, where other websites cannot overclock the card properly, while Techpowerup can. 7850 for example 1050 Mhz for most websites - a joke. Or they cannot do some other simple thing.