What is the meaning of the number in the upper-right corner of the Watchtower screen?

Personally I think the number score is useless for a number of reasons. I think it would make way more sense to use an A-F with +/- for each letter (e.g. A+, B-, etc.). People understand that. And like other said, if someone has rampant password re-use or weak passwords, that should SERIOUSLY drag down the score. Misleading people with false high scores can jeopardize their security.