In other words, you have no solid grasp on the field and so cannot make an effective guess at your ability in the field - so go for middle of the road and that's possibly your best statistical guess...
A prime example from experience: "oh I know development. I use PHP includes all the time".
The biggest humbling exercise I do is to think back 6 months, 1 year and so on. I remember how much I thought I knew and what I produced (some stuff I'm still maintaining too, which is humbling in of itself). Realising how much I've learned between those times and realising I've barely made a dent both shows me how little I know and lights the " I must learn more" fire.
"The more you know about the subject, the more you don't know."
Which explains the upper-quintiles topping out at at 70th percentile mean self-rating.