This can all be thought of quite clearly with a little bit of maths...
NB: All of this assumes perfect conditions etc so might not be totally accurate, but will give a good insight.
Some Maths with Letters
L= Length of track
S = Speed of ride
D = Distance between cars
C = Car capacity
These are the 4 main variables you have when designing / building / operating an omnimover style ride.
NB: Distance between cars doesn't mean the physical gaps between cars. I mean the distance between the same point of different cars (eg: the front of car 1 to the front of car 2).
Using these, you can find:
N = Maximum number of cars on the ride = (Length of track) / (Distance between cars) = L/D
T = Time of ride = (Length of track) / (Speed of ride) = L/S
If you calculate the lengths in meters and the speed in meters per second, then the time T you find here is in seconds.
What this means is that in T seconds, you can load every one of the cars exactly once.
So, in T seconds, you can get a maximum of (Number of cars)*(Car capacity) = N*C people loaded onto the ride.
In one hour, there are 3600 seconds.
So, that means all of the cars cycle round the ride on 3600/T occasions per hour.
And the maximum number of people you can get through the ride in one hour is the number of times all the cars cycle through the ride multiplied by the maximum number of people you can get during the time it takes to cycle through all cars exactly once. This is (3600/T)*N*C.
But we know that T = L/S and N = L/D.
So, you get:
Throughput = (3600/(L/S))*(L/D)*C = (3600*S*L*C) / (L*D) = 3600*S*C / D.
An Example with Numbers
I'll use the details of
Carnval Festival from DRdb (as I can apply common sense since I've ridden it, and haven't ridden any Disney ones)
So here:
L = 240m
S = 2km/h = 0.55556m/s (exactly 5/9 m/s)
D = ?
C = 2 people per car
Distance between cars isn't stated, but the number of cars is listed as 118. Working backwards, that means there's 240/118 = roughly 2.03m between cars (sounds about right to me).
So, the throughput would be: 3600*(5/9)*2 / (2.03) = 1950 people per hour roughly.
DRdb lists the throughput as 1600pph which sounds more accurate. I expect the reason for that is because the speed listed is the maximum speed (which it won't always run at, and it sounds a bit faster than I expected anyways). That number listed could also be a realistic throughput, rather than a theoretical throughput as calculated here.
Why do Omnimover Throughputs Vary So Much?
So a direct answer to your question Matt (as has already been given by others), the key factors which affect throughput are:
-The speed the cars travel
-The capacity of the cars
-The distance between the cars
So, directly at least, the length of the ride doesn't affect the throughput. However, it certainly has an indirect impact.
If you have a huge space for an omnimover, there are loads of design implications. Do you create a physically longer ride? Do you create a ride which surrounds the cars in bigger set pieces? Do you space the cars more to give a more personal experience? What speed will the ride move at?
The same questions, but almost in reverse, occur if you have a small space. Do you create a physically shorter ride? Do you cram as much into a small space? Do you squeeze the cars in as close together as physically possible? Will the ride move very slowly to maximize ride rime?
But it's not necessarily as straightforward as that even. When rides and attractions are designed, there will be at least some ballpark figure for throughput in mind. It should reflect the expected level of popularity, amount of investment, need of the ride, space available, etc. And usually some of those things are intertwined and not separate identities. So if you have a target throughput in mind, the ride will need to reflect that.
For example, if you want a very high throughput omnimover, you wouldn't want to opt for it to have a short length, because that means it will have a smaller number of cars, and so to achieve the high throughput, you need to have a high speed to compensate, which goes against the point of an omnimover.
So, an alternative answer to your question would be "Throughputs vary so much because that's the way the rides were designed". Which is basically the case for every ride ever too.
tl;dr
The speed at which the ride moves is one of the biggest factors for why throughputs of omnimovers vary so much. But also space available, and the creative design aspects behind the ride play a big role too.