I think they're all pretty consistently manufactured. I more tend to think it has to do with over wicking, and leaking out of the juice hole, or, bad o rings, especially in the top cap. I have one right now that will seep a tiny bit of juice out the air hole. Oddly enough, it's the only one with a big conventional silica S wick. My other 4 have micro coil/cotton wicks, and don't leak a bit.
maybe if the juice isn't getting efficiently vaporized it may have a better chance of leaking?

