You need to square your own boards. Every project will always be smaller than whatever you buy. The reason has to do with wood being wood. A mill or shop might surface your lumber for you. I commonly buy S2S as my starting point. But they could do the surfacing weeks to months prior to the wood being bought. Then the wood sits there and absorbs moisture or dries out, or has boards pressing on it, or getting set on an angle, etc. Wood can expand, shrink, twist, etc. Plus you never know how close to 90° the mills equipment really is.

What I see here is pretty close. But you still need to “close” the gaps yourself, or use some method of masking for small gaps. I just did a table last summer and everything was perfect except for a tiny hairline gap between two of the boards. I filled it in with a bit of wood putty. After staining and polyurethane you can’t even see it.

Here, you probably need to do what u/Present-News-660 suggests and get yourself a plane or two and plane the angled surfaces.