Planetary Invasion Always Fails When Garrison is Present.

I picked up GC3 awhile ago, but just got around to playing the game. On my first run of the tutorial I tried to invade the Drengin colony, but every time I did so the last 5 dots of the enemy garrison would become invincible and just slaughter the rest of my legions regardless of numbers. I figured it was a fluke, but after replaying the tutorial and taking out the colony before it had legions I moved on to the first campaign mission. Now I'm in the exact same situation. The enemy homeworld has 5 legions defending it and they become invincible when roughly 3/4ths of their strength is gone. I just threw 40 legions at the planet, the red on the bar was pathetically small and yet, somehow I still lost the invasion. Resistance was also only at ~40%. What am I missing here?