The goal of mixing personalized orchid media is not for the purpose of nutrition for the plants.
Any of the materials used for orchid mixes are either inert or decompose at such a slow rate that they provide negligible nutrients to the plants. The primary source of nutrition for the plants will be from the sun and water, and supplemental fertilizers.
The main factors that are being controlled with these personalized potting mixes are moisture retention and air flow.
Some orchids prefer to stay moist all the time, requiring a mixture that retains water for a long time. Some orchids prefer to dry out quickly, requiring coarse potting media with plenty of air passages. And then there are a lot of orchids that prefer conditions in between the first two.
That, in combination with the grower's personal approach to watering/feeding schedules and local growing conditions, lends to quite a myriad of personalized orchid potting mixes.
The "all-in-one" mixes offered at general stores are a shotgun approach, intended to cover as much of the plants and growing conditions as possible. They may not be the best for your specific combination, but it is a sure bet that it gets the job done for majority of the growers if they choose it.
As you gain more familiarity with your specific orchids and what they want, you can then tailor your own personalized potting mix to achieve the best root zone for your individual plants. Get individual bags of homogenous potting mixes and create your own mix. If you have access to backyard potting supplies, then even better.