Yeah, but the point of having rules for it is so the DM doesn't have to choose on the fly.
Indeed. These rules are suggesting a setup, but the idea is that the DM calibrates this as she wishes and the let the dice roll. Using these rules but ignoring the result means I should have not wasted time with them in the first place.
For instance, if today I bring the PCs into the local woods searching for the hidden entrance of a dungeon, I'm probably choosing 1 hour for turns. Let's say I want the forest only moderately dangerous, and give a 10% chance of monster encounter every turn.
Tomorrow I take them to a trip across the kingdom, and go for 1 day per turn. They'll pass through forested areas but also villages, roads and generally civilized lands with no monsters. I decide that a 50% chance of monster encounter per turn is fine, which may give an average number of encounters per day roughly close to passing a few hours into woods of the previous type.
But ultimately those % are still up to me, I can change them every time even for metagaming reasons, like just wanting to have more/less combats in tonight's game.
If you are looking for creating
consistency in those encounters rates as you switch from 1 hour to 1 day setup for other reasons, you might have to do some calculations to get matching averages. Maybe someone can have fun with that, but I just don't think consistency is a most important goal of these rules. At least the way I see it, there is no fundamental reason why two forests should have the same average encounter rate, and not even the same forest on two different days (especially after a party of PC has passed by, and killed half of those monsters).