I know the individual involved quite well and also know how easy it can be to get things the wrong way around when inexperienced on type.
I had the opposite problem to what most are describing here: I had to circle for a landing once very soon after departure and on late downwind went to put the gear down and found it was already down when in my mind it was supposed to be up. I guess inexperience combined with the fact that most of the circling I had done in training was on one engine meant that I was unable to pick up on the performance not being as it should have been.
Once one has a bit of time on type though, I think that after the checks get missed for whatever reason one not being able to pick up on the numbers not being where they should be shows a major lack of situational awareness.
Perhaps not all types have as obvious a performance differential with wheels up/down as a pa31, but another similar example in my mind is the Emirates incident in Melbourne.
It just boggles me that one would not pick up on the numbers and subsequent performance being that different to standard more quickly, especially by someone with significant experience on type.