> Surely the reproduction of an ancient hardware drum sequencer in today's software should be perfect.

CD-quality is 44.1KHz sample-rate 16-bit stereo. That's 22us per sample. That's a lot of perceptible quantization error when, it turns out, our pitch, timbre and spacial-placement sensory gear is sensitive down to the microsecond level (google "microsecond phase sensitivity" on that). A drum synthesizer done on a PC is limited to the soundcard's samplerate at output, so it's always going to be perceptibly less real than continuous analog.