Wheelbuilding spoke tension vs lateral truing
The rim is probably not made by the very best of rim manufacturers.
I have found that I can create a radially and laterally true wheel where the variation of spoke tensions is around 150-200 N. I use DT Swiss TK 540 rims. DT Swiss is a well-known reputable rim manufacturer and makes high quality rims.
I suspect your rim has not been built to such a high precision.
I think 300 N variation might still be barely acceptable for low-weight riders not riding hard on narrow high pressure tires, especially if you tension the spokes such that the average spoke tension is according to the manufacturer's spec and not such that the maximum spoke tension is according to the manufacturer's spec. Also spoke count helps -- anything below 36, and low tension variation becomes more critical.
For me, I weigh over 100 kg and ride hard on narrow high pressure tires. 300 N variation in spoke tensions would be way too much for me because the loosest spoke would repeatedly go slack and the nipple would self-unscrew, destroying the wheelbuild.