The intricacies of game mechanics

In the current Heroes game, I don't quite understand the turn order. I mean, I know how it's supposed to work in theory, but in practice, it doesn't seem to be happening that way. The implementation of the turn order for units with the same speed on both sides seems to be broken, or maybe I'm just missing something. For example, if I attack and I have a unit with a speed of 18, and the opponent also has a unit with a speed of 18, then I go first. In the second round, the opponent goes. In the third round, the opponent goes again, even though I should be going.