some questions have no answers

I understand that footprints in the sidewalk are from when the concrete was still wet.
I understand that there are different types of cement and concrete.
I understand that sometimes you might patch the sidewalk.
What i don't understand is this

It is a perfect footprint in the sidewalk of a different type of concrete that is perfectly flush with the rest of the cement, as opposed to a raised patch.
Why?
How?
And most importantly, what about my brain constantly causes me to notice things like this?