I think it's the multiple processes going on in the brain, where there's a process that will scan for danger, and this same track is able to break out of the deep thought process. I have to assume this same process just doesn't see those points as something that should interrupt the deep thought process.
The same process with any novel activity will be much more sensitive, but as I do more of the same activity it will consider it a safe activity. The more I do something, the more I would be on autopilot allowing the deep thought process to go on.
For example when I am in a new place, after moving or whatever reason, it is easier in the beginning for me to stay organized because the process is still sensitive and is more careful, but the more I get complacent the less I will be thinking about where to put the things and the deep thought track will be fully prioritised.