AI stack only attacks with one unit?

[CMOS4]

I have noticed this frequently occuring. I'll be faced off against a large AI stack, but instead of attacking with all their units or even many of their units, they will attck me one and then stop. I see no point in this tactic unless the AI was testing to see if could get lucky in the first attack. Since it lost it decided not to continue....I can't say I have ever had this happen when they won the attack, but I am just wondering if anyone else has experienced this.

 

[Dan Quale]

I've noticed it as well, usually it only occurs when their whole stack would simply be slaughtered, when they can win I rarely see the AI show me any mercy.