The Scot was rocked after losing their Premier League title on goal difference last season and vowed it would never happen again.
Even with a healthy lead at the top of the table, and a healthy goal advantage this time on closest challengers Manchester City, Ferguson insisted nothing is won yet.
United let leads slip against Real Madrid and Chelsea at Old Trafford, and Fergie told Inside United: "Where I think we can improve is that we don't seem to kill teams off.
"We've beaten Wigan 4-0 on two occasions but those are the only matches we've won by that sort of margin. In a normal season, I'd expect a few more wins by that sort of distance. Last year, we won 5-0 against Wolves, Wigan, Bolton and Fulham, and we beat Arsenal 8-2, of course.
"We're not seeing those results this season, however. We haven't hit those heights. Hopefully, there's more to come and we'll discover that killer instinct that our teams in the past have had."