Ammo manufacturers don't load all the calibers, all the time. They make occasional runs of a large volume from time to time.
So if they are going to load, say, 250,000 rounds of 30-06, they would look to see if they happened to have a ton of an appropriate powder in stock. Probably not.
So they would turn to one or two powder makers (say IMR and OLIN) and ask what they had available. The IMR guy might say "We have 2,000 pounds of a slow 4350, 2000 pounds of a fast 4831 and 2000 pounds of slow 4064 (all suitable powders).
The ammo maker would then place an order.
I use 4350, 4064 and 4831 as examples that you can relate to. In practice, there are several other powders in that speed range that you have never heard of.
I can recall the powders having modified names, to reflect their speed. Red Dot ( a shotgun powder) was available as Red Dot 1, Red Dot 2 or Red Dot 3. 3 was the slowest. Having a choice was handy, so we could fine tune the wad column to be exactly the height we wanted.