In matchmaking, whether I tap the bumper or hold it, it's not 100%. I've assassinated people by tapping it, and I've beaten people down even after deliberately holding it.
I just chalk it up to matchmaking having its glitches. There are plenty of others as well, such as not being able to change load outs between deaths (It gives me the option to hit Y, but nothing happens when I do), when I get out of black screen I have no armor abilities and no sprint (people also get out of black screen at different times), and after games my spartan will somehow be changed to female.
I've died multiple times because I've assassinated someone who I wanted to beat down