Severus Tobias Snape was a very observant, very meticulous man. He had to be, in his profession. Potions had a tendency to blow up if he wasn't watching the students, and that particular art required exact measurements and a watchful eye. (In addition to that, he was a spy for Dumbledore while acting as a spy for the Dark Lord. It required him to watch everyone around him for any hint that his cover may be blown.) Despite how observant he was, Severus knew he could draw incorrect conclusions in some situations.