Airdrop means there will be some type of task to do and you will get coins/tokens for that. In Airdrop all people who complete task willg et tokens.
Giveaway can be contest where they will choose only few winners out of all entries.
Yes I agree with you, the Airdrop is different from Giveaway in my opinion too. Airdrops are more like Bounty campaigns like social media categories that are required to fill our social media accounts to get tokens.
And Giveaway is like a quiz with prizes that are held not as often as Airdrop and usually Giveaway gifts are big