Here's why some have two 1G resistors, some have one. It comes down to capsule mounting and designer choices.
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The capsule needs to have a polarizing voltage between diaphragm and backplate. Call it 60V for convenience. Shorter to write than "polarizing voltage".
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The input to the electronics needs to be at or near ground potential, but with a very high resistance to ground.
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In a pencil mic, the diaphragm is nearly always grounded because it is connected to the capsule body, which is screwed to the mic body, which is grounded. So a pencil mic has to have both 60V DC and signal attached to the backplate. Therefore, one big resistor to 60V, a coupling cap to keep 60V out of the FET, and another big resistor to hold the FET gate near ground.?
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The OPA Alice and other mics with a headbasket can have the capsule insulated from ground, so neither the diaphragm nor backplate need be grounded. In which case, 60V can be applied to one, and signal can be taken to the FET from the other. The capsule acts as its own coupling capacitor. Personally, after trying different setups, I settled on applying the bias voltages to the diaphragms and having them grounded through caps for audio. Better shielding for hum and RF noise. Take signal from the backplate with a single 1G to ground.
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The downside of connecting the electronics directly to the capsule is greater chance of DC leakage due to humidity, breath, or dirt. Some mics use a DC blocking input cap like a pencil mic just for that reason.
