I thank both (Angus and Herbert) for your help! I have just tried the way suggested by Angus (putting the float table inside a minipage) and it works fine. Probably, my table is not a pure float, since I have selected the "here definitely placement".
This is not a good idea. You need a minipage only for the caption, not for the "float". Otherwise it makes no sense, because the tabular may be of a larger size than the minipage and then the tabular is not horizontally centered.
Herbert
