We shouldn't expect the signal we pick up from the microphone to be identical for every vowel in every environment on every occasion by every speaker. Speakers of a General American accent have a different set of open back vowels to RP speakers. The filtering of the sound source generated by the larynx depends on the configuration and size of the vocal tract. Adults and children have different size tracts as well as different size larynxes. Larger cavities lead to lower formant frequencies.
Larger larynxes lead to lower fundamental frequencies.
Also note that speakers can change their style of speaking in noisy surroundings. Finally, remember that the articulators are not precise mechanical devices and you should expect random variation from repetition to repetition.