The drivers in this family have similar DNA but they are certainly not "literally the same". In addition, you have to think of earphones and headphones as an acoustic system versus just the transducer. There are so many other variables besides the transducer and it's tuning, all which affect the resulting sound (coupling, resonators, materials, nozzle scope, filters, back volumes, and so on, etc). Factor in the compact footprint, and even the most minute difference can wildly influence the output.
A good analogy is an automotive engine, where it's performance can be tuned individually, but the overall experience of driving the car is affected by tires, weight distribution, transmission, feedback, stance, etc. For example the Audi TT and VW Atlas use the VW MQB platform but are entirely different vehicles.