I have something like 110 albums encoded with LAME 3.90.3 -APS. Just out of curiosity I checked what were the highest average bitrates in songs. One song (certainly unknown to you) had a bitrate of 254. AC/DC's Whole Lotta Rosie was close too, at 251.

This made me wonder, what is it exactly what makes the encoder use so high bitrates. Could somebody tell that in layman's terms?

Considering the joint stereo I suspect that one factor is how much the channels differ from each other. What else?

that is a major factor! Also, try --preset standard + LAME 3.96.1 since LAME3.90.3 seems to bloat the bitrate...

Another factor is that the sound/information its trying to compress needs more bits to actually reproduce faithfully when its decoded, i.e. heavy metal music (which seems to be a favourite here on this board ).

Regards

I encoded many files of rock music (including heavy metal) with 3.90.3 and 3.96.1
With 3.96.1 (with the same sources files) the resultant mp3s are smaller.

I find it varies by album. Fuel's Natural Selection is 10% more compressed with 3.96.1 V2 vs. 3.90.3 APS. However, Coldplay's A Rush Of Blood to The Head is 1% more compressed with 3.90.3 than 3.96.1.

3.96.1 seems to struggle much more with classic rock (like The Doors and The Beatles) in terms of compression than 3.90.3.

3.96.1 seems to compress Metal better though.

High frequencies eat bits. Impulsive music also eats bits, esp with APS. APS is tuned to encode high frequencies properly and transients get a lot of short blocks at 320.