This morning a co-worker told me "I try not to be political, but...." and then proceeded to be very political.
Like when people say "No offense, but....." they're going to say something offensive.
"I'm not a racist, but...." Yeah, they're probably about to say something racist.
"I don't mean to beat a dead horse, but...." as they proceed to thrash the deceased stallion once more.
"I'm not trying to be a jerk here, but....."
Not sure what's behind the habit of thinking we can mitigate what we say simply by declaring it's not what it is.
Like Michael Scott declaring bankruptcy
Maybe what people mean is, "I do not wish to make you angry, but I'm about to say something that might make you angry."