Yes, it's a load of rubbish, but it's also somewhat legitimate, in that some mattresses do need a given amount of support to remain firm and stable.

The slats are probably not precisely measured, and may not even be designed for that mattress.

If it bothers you, get a bunkee board or something.

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On my bed it is 3.5 inch apart, land of beds told me the max can only be 3 inch apart. They also said it could damage the mattress and void the warranty.

Is this a bunch of rubbish, why would the bed frame be that far apart if that was the case??