Malbim (here and in Shmuel): Ezra revealed that David did not want merely Pekidah (to know the total number). He also wanted Sefirah (to count each individual), lest he fail to come when David calls to come to war. (b) David also wanted to know the number of children (like the Ramban says; refer to 24:1:3:2), to know how many soldiers he will have when they grow up. All European countries do so nowadays.