In the documentation has wrote that if is enable 'incremental' on tool options, the opacity is full always.

"The incremental check-box does not seems to work as everyone expect. If it is deactivated (the default value) the maximum effect of a single stroke is determined by the opacity set in the opacity slider. If the opacity is set to less than 100, moving the brush over the same spot will increase the opacity if the brush is lifted in the meantime. Painting over with the same stroke has no such effect. If Incremental is active the brush will paint with full opacity independent of the slider's setting. This option is available for all paint tools except those which have a “rate” control, which automatically implies an incremental effect."

So, if you have the paint dynamic with Opacity parameter 'on' this is not true apparently ;-)
Is possible modeling the behavior of opacity with a different gains... for instance, if you use an small rate, around ~10, on 'opacity' slider (tool options).
Then, I am not sure if this is correct or is a secondary effect not expected... so, this is an emerging feature. thanks