Your own thoughts are right, you're not fully extending your hips or shrugging the weight up at the top.
It's a positive and a negative. It's good because you've got enough brute power to hit a weight like that without utilizing the optimal form to increase your power output.
But that exact point is also a negative. You don't want to continue without doing that and messing up your motor skills when it comes to the clean.
Just practice the variations of the pull (i.e mid thigh pull, hang pull and clean pull) and emphasize maximal triple extension at the top of the movement.
Whilst this next point may spark some controversy, note that it is only a coaching cue and doesn't have to necessarily be used:
You can actually focusing on hyper-extending at the hip and almost leaning backwards by a few degrees at the top of the pull.
The main reason being, when focusing on full triple extension, you have to think about the position of the weight with regards to your center of mass. As the weight is being lifted, you can keep the bar as close as possible all you want, but its still in front of your center of mass. At the top of the pull during full triple extension all weight is distributed onto your toes. These two things can cause you to not only be unbalanced (meaning you cannot produce maximal force in the pull), but catch in a position where your spine is no longer neutral.
When you hyper-extend (again I reiterate only slightly) it moves your center of mass backwards slightly and gives you more balance to fully extend at the top of the pull and shrug without the fear of losing your balance (which tends to be the subconscious reason people do not fully extend up onto their toes).
Also, pulls on their own always help. Focus on technique at first and then start moving some heavy ass weight in the pull phases to fully solidify the necessity to fully extend.
Hope this helps.