Saints sign Chase Young to 1-year deal up to 13M (2.7M base, 1.86M signing, 450K workout, 7.99M total gameday roster bonus)

Probably because Young's agent was the one in control of the story and he wanted it to look like you got Young $13 million guaranteed. Reporters can only report what people will talk about and I'm sure Young's agent was out in front on this one saying that it was one year, but it's $13 million guaranteed.
My guess based on what we did with Thomas' contract is that Young's roster bonus may be payable next year, so the guarantee is that he gets paid his roster bonus next year whether he is a Saint in 2025 or not. Here is how Thomas' contract was described:

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You could word Young's the same way, that he is guaranteed a 2025 roster bonus up to $7.99m for games active in 2024. It would be one of those technically one year deals instead of an actual one year deal. We would either need to renegotiate his deal or release him before a trigger date, but in the case of a release he still gets his guaranteed bonus (as Thomas did).