These answers are great. Let me respond to everyone with one post.
Trebor:
Because the fastest attack & release settings of the native Audacity compressor are too slow for speech, e.g. the shortest release-time is one second: the compression takes a full second to wear off, (that’s longer than the interval between words).
I hadn’t thought about that. Thank you for explaining. I don’t know if this is the case though. I checked again, and it’s adding downward compression on long stretches of audio (~7s) where these is only ambient noise, and the original audio doesn’t even reach -20db. And like I said, this only occurs as I raise the Noise Floor even though my ambient nose is still less than the Noise Floor. So, I’m still perplexed.
kozikowski:
you can apply Audacity Audiobook Mastering Macro
Hadn’t tried that. Thanks for the tip. I’ve started using the “Noise Gate” to get rid of my ambient noise (I’m actually removing the noise from cellphone videos to improve the quality on my video projects). I’ve used Noise Reduction a lot in the past. Never the less, I want to master the compressor specifically because it’s so confusing.
(I made another post on it yesterday and discovered the compressor’s example graph is often lying to you, and compression based on peaks always uses upward compression regardless of whether or not you select “make-up gain.” Although, this post is regarding RMS compression for that reason. Why does compressor add gain to things below threshold (Make-up Gain is not selected))
So, I’m still lost as to why a 7s stretch of audio would experience negative compression if it’s below the noise floor, and yet, it does not experience compression if I lower the Noise Floor to its minimal setting of -80 db. Shouldn’t that be the other way around? It should experience compression if it’s above the Noise Floor, not below it, right?.. 
There must be something I’m not understanding correctly here.
And now, to make thing even more confusing- are we all ready for a plot twist?- If I’m using the same compressor setting on ONLY the 7s stretch of ambient noise (~ -45db), the noise selected no longer experiences downward compression at all. If I select the whole track, and I use maximum Noise Floor (still above the actual noise), the ambient noise receives downward compression, but if I only select a chunk of the noise, it behaves as expected, and it doesn’t experience compression at all because it’s below both the Threshold and the Noise Floor.
Boy, this is confusing. Thank you all for the help so far!