They have a random amount of workers and the unit generator decides how many to generate prior to which it will generate.
Early era it's set at a low worker count, low unit count, high chance of shield (tick 1 especially), high chance of being empty.
Towards the middle the worker count rises and rises, the unit count is capped quite quickly, and the chance of being empty is normalized almost immediately after the start, additionally the shield chance lowers.
End of era the worker count settles back down to a normal (but not high) level, the chance of being empty lowers, the unit count suddenly gets a huge buff, the shield chance is pretty much gone.
Essentially it reflects the desired flow: Early era, go for it! So much to get! Brave new worlds! Fight! At tick 100 so much awesome stuff is coming, don't stop playing if you've a bad place now!
Mid era: Grow! Grow! Grow! Keep building! Look, more for you right around the corner!
End of era: Meh. This is pretty much over. Nothing to look forward to, things get harder. Either war or finish the era, stop expanding thanks.