Mp3's are like Limbo dancing, different artists can go lower...
Largely it's a difference in the sonic quality to begin with.
It should be fairly obvious that older recordings from the 60's and 70's can be encoded at far lower bitrates than many newer recordings.
for example if you are ripping and compressing your Creedence Clearwater Revival greatest hits CD, any btrate higher than 128
is a probably a waste of time.
Ditto for early Beatles and Led Zeppelin.
Music recorded in the digital music era tends to be distinguishable
at higher bitrates
(The Second Shinedown album sounds terrible at 192K because of high frequency signals getting lopped off)
Another factor that I personally tend to harp on is the "need" to compress audio files beyond a certain point, because HDD space is "cheap"
For my personal use I save everything at 320K, not because I believe I can hear the difference above 224-256K, but rather because encoding at 320k saves a bit of time and I really don't
need to save more space than reducing the size of the files by
77.5%, which is the savings from going from wav to 320K mp3.
(a Compression ratio of 4.5:1, FLAC only runs a 5:3 compression ratio)