1)
Why did Yo'av bless Yisrael?
1.
Malbim (Shmuel II, 24:3): This hints to one of two 1 reasons not to count them; counting inhibits Brachah and increase, either because Brachah does not rest on what is counted 2 , or due to Ayin ha'Ra.
2)
Why did Yo'av say that they are servants of David?
1.
Malbim (here and in Shmuel II, 24:3): This is a second 1 reason not to count them; there is no need. You think that they will not heed you to come to fight when you call, so you need to count them (refer to 21:12:4:1). You err!


