Confused about how action points work

I decided to attack the black widow spider on the campaign map using a tactical battle.

My sovereign has the fireball spell available, and when I mouse over the spellbook button, it says that casting a spell will cost 2.0 action points.  I have 2.2 action points.

So I open up the spellbook and cast fireball at the spider, and it works fine: nice fire graphic shoots across the screen, and it does damage to the spider.

Afterwards, it's still my turn, and I notice that I now have 0.2 action points.  Makes sense.

However, the game will happily let me cast the fireball spell a second time on this same first turn, and I don't understand why.  I only have 0.2 action points left to spend this turn, and the spell costs 2.0. 

If I move one square (which usually costs 1 action point) it will allow me to do that as well, and then my action point total reads 0.0 and trying to open the spellbook results in a message to the effect that I do not have enough points to cast any spells.

Am I misunderstanding the entire idea of how action points work?