Yeah, it's simple math, more or less.
The standard is ten chairs per minute , or one chair every six seconds - this is the limit in Colorado according to state law. Most other places follow this standard, although I know of one chair that runs at 12 chairs per minute, and many lifts are run at a lower rate.
Number of chairs per minute times the capacity of the chair times 60 will give you the (theoretical) uphill capacity of the lift. Note that the rope speed is irrelevant - that is, a detachable quad running at over 1000 feet/minute will have the same uphill capacity as a fixed grip running at 350 feet/minute. A quad char at 10 chairs/minute will have an uphill capacity of 2400/hr.
In practice, detachable lifts have fewer misloads (stops) and fewer empty seats so their real-world capacity is slightly better than a similar fixed grip by about 10%.
I have no idea how they arrived at the 25% figure. Going from four seats per carrier to six seats per carrier should give a 50% increase, with a bit more in practice if the six pack is a detachable replacing a fixed grip quad.