Because in past Pensieve experiences, Harry has often noted things that the owner of the memory hadn't actually noted or registered himself - walking away to go look at his father and Sirius during Snape's memory of an exam, for instance. And the simple fact that Dumbledore bothers with a Pensieve at all, despite having a rather good memory, has to mean he can find out more that way - see things precisely as they happened, instead of having to rely on his good but imperfect memory.

I suppose there could still be differences between various people's memories, but as Harry doesn't seem to note any, I'm not sure we should conclude that there are any.