Thank you for the feedback!
The selection of the attack and movement patterns of each stage of every boss is not entirely random, but still based on some randomness. Maybe you got unlucky with the first one or especially lucky with the others, or you were faster in getting the weak spot, which also would be fair enough. Either way, it's actually not intended to scale up that much when fighting the bosses on their own, they are designed to be pretty simular in difficulty. The final fight however is supposed to be way more difficult, as you know each of the bosses by then.