The first person killed by the Predator's shoulder cannon (aka plasmacaster) was Blaine. He is shot from a far distance and hit in the back, which leaves an exit wound the size of a basketball in his chest. There is something of a continuity error at the moment he's hit; a splash of blood hits him in the neck and back of the head. One theory is that the Predator initially fired a different weapon that missed Blaine, and the Predator then switched to the Shoulder Cannon. In scenes where characters are supposed to be splashed with fluids like blood, a small gun of sorts (often nicknamed a "guacamole gun") is placed off camera and the fluid is shot onto the actor or stunt person. The production crew & director probably shot the blood onto Jesse Ventura thinking they'd shoot further enhancements to the scene showing a different action taking place.
Next killed was Mac, he is shot at close range in the chest (continuity shows him shot in the head, but his body is shown to have his head intact and his chest with the wound) the wound in his chest is relatively small, we never see the exit wound. Third hit with the cannon is Dillon, the Predator is perched on a tree branch and fires at Dillon, hitting him in the arm, completely severing it. Finally, Poncho is shot in the left side of the forehead and there seems to be very minimal damage, though he is killed, there is no exit wound, and there's minimal blood shown. It's possible that the predator, being so close to Pancho and wanting his skull for a trophy, reduced the power of the cannon so as to cause less damage.
Then Dutch is hit in the shoulder, if one watches the scene in slow motion you can see that the blast hits the M16 assault rifle which Dutch is carrying, the gun can clearly been seen being blasted in to two separate pieces. There is a large burst of blood from his arm, he is knocked backwards, yet we see he has a minor 3 inch cut on his arm. So it's possible he was hit in the shoulder by shrapnel. Some theories are: (1) The Predator's weapon has different charged bursts. The longer it charges, the more powerful the damage. (supported in many of the video games), and (2) The energy emitted from the cannon gathers more energy as it travels. The farther the distance it has to travel the more powerful it becomes. (supported by the larger wounds being suffered when fired from a farther distance). Note, however, that the concept of what exactly constitutes the Predator creature was changed part way through production. The original creature was shorter and more of a cross between an insect and a lizard that carried a spear gun-like weapon that can be seen hitting Blaine in the shoulder right before he turns and is shot in the back. Much of the film was shot with the original creature in mind, so this could explain the inconsistencies in the wounds.