There's an odd bit of numerology that one man came up with where he assigns air, earth, water values to numbers. 7 is in the place where earth and water meet - so you get mud. He was constantly accusing people wtih a lot of 7's in their name as being lazy good for nothings LOL! Since I have two 7's in my name I resented that. ;)
Anyway, apart from that, 7's love to analyse things so deeply that their words come from a very different place that mercurial people. 5's and 3's often don't know what they're going to say when they open their mouths! ;) But a 7 always knows and probably analyses it three different ways before they choose one too. ;)