You TRY to focus on your final exams.
You really do.
But, in the middle of them, you suddenly relate them to the O.W.L.s
Which makes you think of when the Marauders had to take their O.W.L.s
Which makes you think of when Snape called Lily a Mudblood.
Which makes you remember that Harry saw this memory, resulting in him being kicked out of Occlumency.
Which eventually leads him into dreaming that Sirius is in trouble.
Which, through a series of events, leads to Sirius’s death.
And how could ANYONE possibly think about quantum physics, when Sirius is dead?