What's the difference between T9.9 and T15?

What's the difference between T9.9 and T15?

LAOGEN 9.9hp and 15hp outboard motors are from the same product development platform. The same specs for T9.9 and T15 is same in size, overall structure, engine and propulsion device. The difference between them is power and fuel consumption.
For example, T9.9 and T15 are same in lenght, width, displacement, engine type，gear oil capacity，etc. But the output power is different.