Actually, Yann, that's only part of why brushed clear flats tend to be glossier than airbrushed flats. While this does occur to a certain extent with some flat colors, it is most noticeable in clear coats. There are two reasons for this, other than the one you gave: 1. clear flats contain only a single flatting pigment, usually a form of microsilica. 2 a brushed coat is thicker than a sprayed coat.
Brushing a clear flat tends to pull the pigment away from the paint surface, and the thicker coat takes longer to dry. This last allows the pigment to settle slightly, resulting in a higher amount of the binder (which is inherantly glossy) at the surface. Color flat coats usually contain several flatting pigments, some of which are much more resistant to settling and segregation than microsilica.
The exception is flat white, which may not contain anything other than a white pigment. That is because titanium dioxide, the most common white pigment, comes in two forms, one of which produces a very good flat surface without the addition of other flatting agents.
To produce a flatter surface with a brushed on flat coat, try diluting the flat before brushing it on, preferably with a compatible solvent that evaporates rapidly, and apply several quick coats. Try to cover with a single stroke—don't go over an area you just painted until it has cured. This technique takes practice, but it can be done.