Ran out of gas even though I didn't

Hi all,

I'm new to cryptocurrency's and NFT's and Ethereum and the likes and have just attempted to purchase an NFT on OpenSea. I set the gas limit at 200,000, after looking at a similar transaction on the EtherScan website, which showed a max gas limit of 250,000 but only used 170,000. The transaction carried on for 10 minutes and 30 seconds before coming back and saying it had failed. It says the reason was due to me "running out of gas" even though it used 98.78%. If it didn't use the whole amount, why is it saying that I ran out of gas? I attempted to purchase this NFT last night and only used 40,000 gas as I didn't understand at the time how gas worked. I felt like I had an understanding this time, but apparently not. I don't want to try and purchase this NFT again without fully understanding what's happening, as it seems to just be throwing money down the drain. I've read up about 'slippage' as well as was wondering if this could be the issue?

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Would greatly appreciate any knowledge on this topic to help me understand what has happened and what I should do going forward.