I've noticed this when speaking about complex topics. The unvocalized thought will be a the edge of my brain, and while, with simpler topics (such as this), I pre-vocalize it internally before speaking, with complex topics, I am unable to, and instead, the final vocalization just happens unconsciously without any internal "rehearsal".

Some insights I even have trouble putting into words, and I stumble for a bit trying to explain them to co-workers.