They are designed to pick up the vibrations produced from vinyl LP's, which were far weaker than the massive vibrations produced by playing an old acoustic record. Stereo LP's use both lateral and vertical modulation, on the record surface. In other words, one channel is 'shaking', while the other channel 'bounces'. Modern cartridges are sensitive to vibration in both directions. Victor and Columbia type 78's only use lateral, or 'shaking' modulation.
Any vertical modulation is probably only noise.
The 4A reproducer is designed only for lateral modulation, and is very immune to anything else, so the result is a quieter recording.