There are a number of analogies to use, but I'm lazy, so here...
From another site:
Uncompressed audio from standard redbook CD has a fixed bitrate. It is stereo (two channels), 16 bits per sample, 44,100 (44.1 kHz) samples per second.
2 x 16 x 44,100 = 1,411,200 bits per second (bps), or approximately 1411 kbps
But when compressed using FLAC or another lossless codec, the bitrate will be less. A particular track that compresses 30% will have a bitrate of 70% of 1411 kbps
30% compression -> (1 - 0.30) x 1411 = 988 kbps
and so on...
40% compressed -> 847 kbps
50% compressed -> 706 kbps
60% compressed -> 564 kbps
65% compressed -> 494 kbps
70% compressed -> 423 kbps
In my own FLAC library I have files that have compressed as little as 21% and some as much as 76%. So, yes, the bitrate can be expected to vary wildly.
And...
Go to random.org and generate a text file from random characters (10000 strings of 20 characters, using both numbers caps and minuscles) file. That gives you 10000 lines, each of 20 characters. Save it into a .txt file.
Compress it into a .zip file.
Then create a text file consisting of 20000 lines, each of 20 characters, where all characters are space. That is the same size as the previous .txt file.
Compress it into a .zip file.
Both zip files will be unzipped to their respective originals, yet the latter is much smaller. Is that because it is of lesser quality?