Please don't show me that like I'm wrong. :) The sentence on there clearly says "He seems like a bit of a knob" which gives a totally different indication.
In this case (which is more specific than your question) "seems" itself shows that the speaker doesn't know the person well and is just basing their words off of what they have seen. Like I said; context.
"He's being a bit of a knob," or "He's being a knob," are statements which don't necessarily indicate that the person knows the "knob."
Knob is a very vulgar word so "a bit" is just used to soften its harshness / certainty. (known as a "hedge") So yes, this one would be used more in cases where we don't know the person well OR if we're not sure that their actions reflect on their true intentions, or in other words; we simply don't want to judge them too harshly. (But doesn't apply to everybody all of the time :) )
*talking about a stranger* Person 1: He seems like a bit of a knob. Person 2: I know...
*to each other* Friend 1: You're being a bit of a knob. Friend 2: I know, problem?
I was giving you the meaning of the actual word rather than how to use it. :)
Hope this makes sense and sorry if I came out as rude in any way.