Funny, I noticed the same thing earlier (well kind of). I was looking at the stacks/ bubbles my perc on my Zob makes. And it only pulls off of one section of the perc holes when I hit it slow, but when I pull hard it goes through more/ all of the holes.
Hit your bong in the mirror with no smoke in it and see how the bubbles stack up. You'll probably be surprised, I know I was and will never by this style of water pipe again