I could look it up, but I think this illustration should work:
Let's say the undead in question has 13 HD. Clarissa (18 lvl paladin) can turn anywhere from 28-38 HD of undead with one of her attempts. Let's say six attack her. She'll never be able to turn more than two of them. Why? Because 13x2=26, but 13x3=39, which is one higher than the maximum number of HD she could possibly turn.
Now, the strongest undead Clarissa has a chance to turn is a 20 HD undead, and she could only hope to turn one of them. However, anything below 12 HD she'll turn automatically, until the total number of undead HD from the first roll is exhausted. But that's a spread.... she has a chance to turn anywhere from 12-20 HD undead. A cleric of the same level (18), yet with no charisma mod, will be able to turn 14-22 HD creatures.
Now, let's say Clarissa gets lucky on the first roll and affects a maximum amount of undead HD - 38, in her case. But she gets unlucky on the second roll, and can only affect 12 HD undead creatures. This means she wouldn't be able to turn a single 13 HD undead, despite being able to affect a total number of 38 HD. A cleric of her same level would, however, be able to affect a 13 HD undead automatically.
Now, Clarissa, the clerics you may have seen who seem to be doing a better job than Clarissa may not be able to turn as many HD at once that she can turn, but they can potentially turn more powerful undead, especially if they have the sun domain, as already mentioned.
This is a direct illustration of what Weeblie stated. It is an application of what can be found in LORE and the explanation s0ulz gave.