It has to do with the characteristic of the beam. The positrons are held by a magnetic field that prevents is from disassociating. The magnetic field ceases to hold the particles when it hits something, int his case a ship. However, the particles need time to disperse, so the ships doesnt blow up all at once. It will penetrate into the ship, then disperse inside it, causing a non-instant internal explosion.
Tanhauser, however, simply consumes its target since the beam it firse is very thick. Or at least it was when it was fired at the girty lue. It seemed to get weaker and smaller everytime it next appears in the show -_-