Why does the verb "haber" sometimes not match the person and number of the subject in a sentence?

The main uses of the verb "haber" include its use as an auxilliary verb (ha comido) and as part of a verbal periphrasis (he de soportar), in this case, the person (first, second or third person) and number (singular, plural) do match those of the subject in a sentence.However, one of the most important uses of this verb is in its impersonal form. This means that "haber" in its impersonal form appears in sentences that do not have a "person" or object as a subject. For example in "Hay una té en la mesa", one may think that "té" is the subject when in reality it is a direct object. In another example, "Hubo varios voluntarios en la sala", it might appear more confusing, as one might think that the subject would be "varios voluntarios", which is in plural.One way to make sure whether the verb we are working with is impersonal or not is to attempt replacing the apparent subject with a direct object pronoun (lo, la, los, las) and a subject pronoun (él, ella, ellos, ellas), and check with which one is the sentence coherent. In the sentence "Hay té en la mesa":Él hay en la mesa (as subject)Lo hay en la mesa (as direct object)As we can see above, the first sentence is not coherent as a "person" or object cannot perform the action of the conjugated verb "hay". Therefore, the second sentence proves that the verb "haber" in this case functions as impersonal.