If you consult your periodic table, a totally normal thing we all do all the time, you'll see that phosphorus has one very different trait than most macrominerals- a negative ion. This means that phosphorus frequently comes along with minerals like calcium or potassium, even if phosphorus isn't needed in the formula. These sources are usually the best in terms either flavor, solubility, price, activity level or a combination thereof. This makes phosphorus the french fries of the macromineral world in a sense- sure, you only want the sandwich and a drink, but it's usually cheaper and easier to just get the combo meal. This also means that it can be difficult to limit the phosphorus in a formula while maintaining the same price and quality. As formulators, we frequently must face the choice of either reducing the phosphorus to meet the desired label claim and sacrificing another trait or raising the label claim for phosphorus. The latter may seem preferable, but if the product's marketing keys on exactly 10% DV for all the nutrients (for example), it's not so easy. At the end of the day, there is no one right choice, but rather a right choice for each individual product's situation.
Unnecessary fast food metaphors aside, phosphorus has other formulating considerations- and these ones are shared with its macromineral brethren. It is extremely important to factor in background nutrition from sources already in the formula for macrominerals. Even a few %DV can save hundreds of milligrams in usage rate for the final premix. This will reduce cost, improve the final product's flavor/texture, and saves room in the formula for any number of useful ingredients.
As with any ingredient, there are innumerable general rules to follow, but every product is different. Identifying the best source for your product, whether it's phosphorus, potassium or water, is more specific than any general rule. But keeping the above in mind is a giant step in the right direction.