I get a lot of 你 and 我 too. It seems the Popup Review counts every instance of 我 as a separate item with its own review history. Since I study a lot of lessons, I have 15 我 flashcards to review for every one 太阳黑子活动加剧.
It would make more sense to count 我 only once, no matter how many lessons we've studied it in. Might tricky to implement, given that some words have multiple definitions. But it would keep us from reviewing the same characters over and over again, or manually deleting words.
IIRC the Skritter guys have this in their SRS. You can add characters from multiple textbooks, but a character, once added, won't be added again.