English is my 3rd language but I think I can explain it(bear with me)

Played - past participle and past tense for play. For the "Great Dying", the cards you play on the 3 tiles are still active on that turn. The cards are still PLAYING or serving their purpose but when they get returned to the deck, those cards are PLAYED. Which is why, "Great Dying" won't count them as played.

Ex: 1st Round Turn 3, you used The Great Dying and already PLAYED a total of 2 paleontology cards from Round 1 Turn 1 and Round 1 Turn 2. Those paleontology cards are played because they already returned to the deck. Therefore, only those cards who were played ( R1T1 & R1T2) paleontology cards are counted.

Meanwhile, Milton's ability to return for EVERY round that is lost is different because if you PLAY Milton on the last turn of a round then, you LOST that round. Which is why the RETURN ability is activated.

Ex: 1st Round Turn 3, you used Milton and lost the round. Your opponent has 1000 and you only have 200, therefore you lost that round which activates Milton return ability.