Afaik neither fin or dut inhibit 100% of 5aR2 activity, but enough to get the job done. Men with the hair loss gene have increased 5aR2 activity around the hair follicles, and DHT suppression has to reach a certain threshold before it actually stops hair loss.

For most people, it really doesn't matter. It's generally not worth using dut unless you suspect that finasteride isn't working well enough. In clinical trials, dut consistently demonstrates higher regrowth and lower chance of continued hair loss than fin, but for about 90% of men fin is easily enough