Why did the time machine work different for J?

It could be called plot inconsistency but I rather not think of it like that.

       0   o
      []- '|| J_prev at t = -5
      /\   /\
      =========================== Ledge
            ^
            |---- Where J current will end up after jump at t = -5







o--== J current at t = 0           __
 '
                                     2ft limit
================================== __

Now J decided to go back a few minutes to when Boris just threw him, this would place him exactly at the stop where J_prev would be at. Without trying to get too deep into space-time continuum, the time device moves you by time not space. So you are in the very exact spot, two steps (plus or minus) before you made the jump.

Then, one could see the situation as two bodies being at the same exact spot at the same exact time. So there can only be one mass that can occur at that spot.

Building on that, J uses the device to jump the timeline so in order to maintain consistency without breaking anything after he finished saving the world with K he would jump back to the time he left (on the building). If he were to jump again while not returning to complete the first jump in 2012, which timeline would he be in? A paradox would occur in my opinion. The most logical thing for the writers to do would be to have J enter his old self on the second jump.

J could let go of Boris when they were falling and travel a few seconds/minutes into the future instead, only if that spikes weren't poisoned, making him bleed badly. Boris would land hard (not stopping few feet above the ground) and when J would arrive in the future a few seconds/minutes later he would be the only one on the ledge.

Or when he travels a few seconds back in the movie, he could just push Boris of the ledge since he appeared where he started the jump (next to boris, practically hugging him) and since J's past self would still be laying there, not being hit by spikes, the future J's spikes would disappear too. That could leave us with two Js or when the time that passed from when J got up and started running towards Boris till he jumped back in time would pass future J would disappear since past J wouldn't need to make the jump since Boris wouldn't be on the ledge anymore.

Yeah, I guess movie makes it a bit illogical but I never really thought about it before few minutes ago. And it appears that both J and Boris travel few seconds back but only J remembers that it already happened. I guess because he broke the laser line but they still both traveled because they were touching. :)