Those native speakers use them indistinctly, the rule says that: Quizá must be used if the following word starts with a consonant, and Quizás must be used if the following word starts with a vowel. Quizás ella sabe la verdad. Quizá nosotros sabemos la verdad. I hope this helps.

Well, I have to disagree with Mario. Both words are used indistinctly, no matter what the following words begins with. You can see this on the DPD (Diccionario Panhispánico de Dudas): http://lema.rae.es/dpd/?key=quiz%C3%A1 In fact, have a look at the examples! So don't worry about using one or the other, both will be OK!