In Survival Mode (Endless), do the zombies on each wave vary?

Is the type and number of attacking zombies in each wave predetermined? Or does it vary depending on factors such as how many plants I've got in the ground, what strength these plants are, or how much Sun I've saved?

I've played dozens of rounds and zombies definitely vary, but as I use very similar strategy each time it makes me think that the variation is rather random.

The zombies for each wave are a little random, but are mostly based on the wave number that you're on.

I played several rounds with minimal cheap plants versus and aggressive and expensive strategy, and I found no difference in the type of zombies which attacked.