It's most of the time 12.01, because nobody is sure whether midnight is actually 12:00 a.m or 12:00 p.m. To avoid confusion or phishy interpretations they choose the one minute delay, cause it's clear that way which time was actually meant.
Midnight is obviously 12:00am, but people don't know which day it belongs to. Is midnight the start of the day? Or the end of the day? Tuesday 12:00am could be 2 different times (spaced 24 hours apart) depending on your interpretation. If you say 12:01am, there is no ambiguity.
A lot of people in high school probably ignored their math teacher when he explained open/closed intervals.
