Every VBR preset in Nero's AAC encoder has an associated "estimated bitrate range" (or at least that's what I guess). My question is, based upon wich criterion were these rules elaborated?.

Nearly every song I have encoded with a given preset doesn't match at all these estimated bitrate ranges. Almost always the average bitrate is way superior to the expected range.

For example, many songs encoded with the "Streaming, 100-120 Kb/s [LC AAC]" preset reach average bitrates around 170 kbps. Also I have some files encoded with the "Transparent, 110-150 Kb/s [LC AAC]" preset, whose average bitrate goes beyond 200 kbps, sometimes reaching 220 kbps.

My interpretation when I look at a preset like "Transparent, 110-150 Kb/s [LC AAC]" is that the resulting average bitrate will be of 110 kbps as a minimum and 150 kbps as a maximum. And well, 220 kbps is far from that.

Please note that this is in no way a rant or anything like that, just a minor constructive opinion about a (imo) great encoder.

I think that's a known problem. Updates to the aac encoder didn't include necessary changes to the estimated bitrate ranges.

The estimated bitrates are due to be updated in a future release of Nero.

Could it be that it encodes on a channel basis ? (preset 110-150 -> 220-300 kbps for stereo, minus a few kbps ?!)

The bitrate is not expressed on a channel basis...

I believe it has more to do with Jerethi´s post.

Jerethi is correct.

An update is coming that (hopefully) reduce the bit rates as well, once the developers can give time to tuning the high end pre-sets. Even then, the bit rate setting statements will still likely need to be updated.