Here's a line graph of the bitrates in my test. It shows interesting variation from a track to another. Naturally, it doesn't tell how the bitrate varies inside the files in individual frames, but seems like the encoders don't always agree with each other when they estimate the needed bitrate. For comparison, I included LAME -V5.
The tested tracks are:
Various
1 AC/DC - Highway to Hell
2 Adiemus - Boogie Woogie Llanoogie
3 Barry White - Sho' You Right
4 Björk - Possibly Maybe (Lucy Mix)
5 Davis Bowie - Starman
6 Dido - Life For Rent
7 Duran Duran - Astronaut
8 ELO - Livin' Thing
9 Erich Kunzel & The Cincinnati Pops - Theme from The Pink Panther
10 Evanescence - Going Under
11 Faithless - Mass Destruction
12 Garbage - Bleed Like Me
13 Jamiroquai - World That He Wants
14 Kraftwerk - Tour de France Etape 1
15 Morrissey - Irish Blood, English Heart
16 Paco De Lucia - Rumba Improvisada
17 Santana - Oye Como Va
18 Simply Red - You Make Me Believe
19 Sting - Desert Rose
20 The Beatles - Let It Be
21 Tina Arena - Symphony of Life
22 U2 - Vertigo
23 Whitney Houston - Queen Of The Night
24 Yello - Planet Dada
25 Yo-Yo Ma - Libertango
Classical
26 Aldo Ciccolini - Satie, Sports Et Divertissements, Le Flirt
27 Alfred Brendel - Beethoven, Piano Sonata No 15, Op 28 Scherzo
28 Baroque Festival Orchestra - Vivaldi, Sinfonia in C major, 3. Presto
29 Berlin Philharmonic Orchestra - Mozart, Requem in D moll K 626, Sanctus
30 Berlin Philharmonic Orchestra - Strauss, Also Sprach Zarathustra (Thus Spoke Zarathustra)
31 Berlin Radio Symphony Orchestra - Mahler, Symphony No 8, 2 Chailly, Ewiger Wonnebrand
32 Christophe Rousset - Farinelli, Il Castrato (OST), J.A. Hasse, Generoso risuegliati
33 Concentus Musicus Wien - Bach, Matthäus Passion BWV 244, Da ging hin der Zwölfen Einer
34 Daniel Barenboim - Mozart, Piano Concerto No 3 in D major, K40-3
35 Gérard Lesne - Vivaldi, Sonate Op 2 No 3 pour violon & bc, III. Adagio
36 Giuseppe Sinopoli - Elgar, Cello Concerto, Serenade for Strings, Enigma-Andante
37 Itzhak Perlman - Paganini, 24 Caprises, No 1 In E
38 Jascha Heifetz - Sarasate, Zigeunerweisen, Op 20 No 1
39 Jessye Norman - Angels we have heard on high (Trad.)
40 Kirov Orchestra & Chorus - Khatchaturian, Gayaneh, Säbeltanz
41 Leslie Howard - Liszt, Douze Grandes Études, S 137 No 1 in C
42 London Sinfonietta - Saint-Saëns, Le carnaval des animaux, Hémiones
43 London Symphony Orchestra - Ravel, Daphnis et Chloé, 10. Tres modere
44 Marie-Claire Alain - Bach, Wo Soll Ich Fliehen Hin BWV 646
45 Michael Nyman - The Piano OST, A Bed of Ferns
46 Orchestre Symphonique De Montreal - Elgar, Enigma Variations No 1
47 Philharmonia Slavonica - Bach, BWV 1067 Rondeau
48 The Cleveland Orcestra - Ravel Valses, nobles et sentimentales, 1. Modere
49 The Philadelphia Orchestra - Tchaikovsky, The Nutcracker, Op 71a (Ballet Suite) No 2
50 Zbigniew Preisner - Trois Couleurs Bleu (OST), First flute