Title says it all; how reliable are the enemy force predicitons? How reliable are the estimates for neighboring a province vs with a spy? Will a spy ever lie and indicate a smaller force than there is in actuality?
Keep in mind also that the scout is just reporting what is there currently. Movement in friendly provinces happens before moving into enemy ones... so the enemy can drop a big stack onto it in the same turn you try to attack. They can also recruit units there too.
I lost a battle in which the enemy had 113 surviving stealthy glamour units. I had a scout in the province. The report only mentioned the nonstealthy units. It reported just over 50 units when I could look in the messages and see 160 surviving nonrouted units at the end of the battle.