My thought process was that the park probably aims for a maximum “palatable wait” for RAP users.
My assumption was that the ratio was likely to be higher when the demand was higher.
This is because if there is a lower demand-to-supply ratio (i.e. a quieter day), then the ride doesn’t have to process as many RAP users in a given period to keep to that “palatable wait”, therefore the percentage of throughput allocated to RAP can be lower.
Whereas when there is a higher demand-to-supply ratio (i.e. a busier day), the ride has to process more RAP users in a given period to keep to that “palatable wait”, therefore the percentage of throughput allocated to RAP needs to be higher.
This is of course assuming that throughput is the same on both days, which isn’t always the case, but my point still stands.
I could be completely wrong, but that was my assumption. It would explain why the most popular rides (in the context of Alton Towers, I’d say these are Wicker Man and Smiler) and the low throughput rides often tend to have greater percentage of throughput allocated to RAP. In a scenario where demand is a higher percentage of supply, then you need a greater percentage of throughput allocated to RAP to deal with the added demand.
They probably have to prioritise RAP over the main queue because a typical user of the RAP queue would be unlikely to understand why the main queue was prioritised over the RAP queue, whereas a typical user of the main queue would probably understand why the RAP queue was being prioritised over the main queue.