Broken English

There can be few of us who don't encounter some quite shocking misuses of the English language on a daily, if not hourly, basis. Unless you happen to live in a country where English isn't the first language, of course. Generally speaking I don't have a problem with those people who choose to pepper their sentences with, for example, innit. If I did have a problem with them then I'd spend most of my days coiled up in tightly wound ball of rage and I don't have time for that. I didn't even mind when I heard the following on the bus last week:

"Fucking I'm going up the 'ospital innit. My mum's there. Fucking don't know what's fucking up with her innit."

Terrible, eh? But I understood what was meant and, for me, that is what is important. I'm not a fascist and I don't believe that it is my place to tell other people how to communicate. There is, however, one particular phrase that that makes me want to hack off lumps of my own flesh whenever I hear it. That phrase is: I'm not being funny, but...

What the hell is that? And how did it enter common parlance? What does it mean? I find that the phrase is often followed by a rhetorical question and I have no idea why - here are a few examples:

I'm not being funny but who does she think she is?
I'm not being funny but how did an ugly fella like him get a fit girl like that?
I'm not being funny but how does he afford to go out that much on what he earns?

The first few words of each of those sentences are completely superfluous yet I hear them many times every day. Today, I sat and listened to two of my work colleagues have a conversation about someone they didn't particularly like, each of them beginning their sentences with I'm not being funny but. I have a friend, an English graduate, a journalist, a man of letters (loosely speaking) whose words are read by people up and down the country every day. He says I'm not being funny but... on a regular basis. Why? I don't get it. If anyone, apart from me, ever reads this (unlikely, I know) and has any idea as to why this phenomenon occurs I would be grateful if they could tell me. Thanks.