X264 bitrate warning

[wyti]

Hi, i'm using x264 rev 1090 and after starting a 2 pass encode I get this warning :

x264 [warning]: target: 835.00 kbit/s, expected: 823.17 kbit/s, avg QP: 19.5503

what does this mean ? why do i have this warning ? And what to do ? (the decrease in bitrate isn't really important, but i'm curious)

My settings :

x264 --keyint 1500 --min-keyint 2 --bframe 2 --b-adapt 1 --b-pyramid --ref 2 --deblock 1:1 --bitrate 835 --vbv-maxrate 1500 --vbv-bufsize 1500 --qpmin 1 --qpmax 51 --aq-mode 0 --aq-strength 0 --pass 1 --stats x264_stats.log --qcomp 0.66 --direct auto --me dia --subme 5 --psy-rd 0:0 --8x8dct --progress --thread-input --threads auto --non-deterministic -o %Sortie% %Entree%

x264 --keyint 1500 --min-keyint 2 --bframe 2 --b-adapt 1 --b-pyramid --ref 2 --deblock 1:1 --bitrate 835 --vbv-maxrate 1500 --vbv-bufsize 1500 --qpmin 1 --qpmax 51 --aq-mode 0 --aq-strength 0 --pass 2 --stats x264_stats.log --qcomp 0.66 --direct auto --me dia --subme 5 --psy-rd 0:0 --8x8dct --progress --thread-input --threads auto --non-deterministic -o %Sortie% %Entree%