Why can I hit notes more accurately on humming and vowels than on words?

There’s a complex relation between resonance rooms and vowels:

Good singers have to learn to control and refine their vocal sound on a completely new level. They must learn to use the enormous potential of resonance with a new process in a targeted manner in order to produce volume in a relaxed manner and to articulate it optimally. In addition, they get to know a way to improve the intonation by means of selected vowels and to decide whether they want to merge homogeneously into a choral sound in an ensemble or sound out as a soloist.

Spoken words or lyrics are disturbing our sense for pitch level as we usually don’t speak like we sing.

A good response is crucial in order to minimize the risk of forcing. So far, resonance has mainly been conveyed through visual language and subjective perception.

There is a specific vowel-resonance diagram that clearly illustrates the complex relationship between pitch, vowel sound and resonance. With the help of the diagram it is immediately clear why some vowels do not "work" with certain tones. This knowledge can accelerate the learning process enormously.

Thus a singer can control the resonances with the vocal tract articulators tongue, lip opening and throat position much more precisely than normally. It is then even possible to use vowels in such a targeted manner that pure intonation emerges completely naturally and relaxed in the ensemble sound. By such training ensemble singers will experience homogeneity and harmony on a new level.

https://www.researchgate.net/figure/The-variation-of-vocal-tract-resonances-R1-and-R2-lip-articulation-and-sound-pressure_fig15_44675402