Yes, it's because, as I mentioned, Replace has two separate instructions. Same with Summon, Reap and Hill Fort. Blockade on the other hand is like Armory.
Summon could have been like Blockade too, but then it would have to specify that "if you did" referred to setting it aside, not gaining it: "Gain a card costing up to $4, setting it aside. If you set it aside, then at the start of your next turn, play it." Same with Reap. *
Replace would be even trickier, maybe like: "Trash a card from your hand. Gain a card costing up to $2 more than it, putting the gained card onto your deck if it's an Action or Treasure. If it's a Victory card, each other player gains a Curse."
Hill Fort: "Gain a card costing up to $4, putting it into your hand if you choose to. If you don't choose to, instead get +1 Card and +1 Action." (Also it would remove "choose one", breaking the interaction with Elder.)
It's pretty clear that the existing card texts are for the sake of brevity and clarity. The fact that they entail two different instructions is probably just a side-effect.
* With the new rule that you can't play cards that are lost track of, we could actually drop "if you set it aside", but that rule didn't exist when Summon and Reap were published.