In English, when a stressed syllable is followed by two (or more) unstressed syllables, the vowel immediately following the stressed syllable is usually dropped in colloquial/fast speech (not every accent/dialect).
It is called elision.
Elision is defined as the deletion of one or more sounds in a word. The sound may be a consonant or a vowel.
- When t comes between two consonant sounds, it's often elided. Can't think [kɑ:nt θɪŋk] in normal speech would be pronounced as [kɑ:n θɪŋk] (try both when you're speaking fast). It's for the sake of ease.
- Comfortable is pronounced /ˈkʌm.fə.tə.bəl/ in slow speech but in fast/casual/colloquial speech, it's pronounced /ˈkʌmf.tə.bəl/.
Elision is a normal speech phenomenon and comes naturally to native speakers of the language in whic it occurs.
Moving to OP's question:
There are lots of words in which the vowel is dropped when it occurs in an unstressed syllable immediately following the stressed syllable.
Examples: Comfortable, temperature, family, vegetable, chocolate, every, nursery, average, business, evening, favourite, interest, general etc.
Presumably, the vowel following the stressed syllable was once pronounced but then it got reduced for some reason and eventually dropped (syncopated). This is a special kind of elision known as syncope.
Syncope is the loss of one or more sounds from the interior of a word, typically the loss of an unstressed vowel (often [ə]).
In the above words, it's post-tonic syncope.
Post-tonic syncope is only permissible in English if the schwa [ə] is followed by a single consonant and an unstressed vowel as in comfortable /ˈkʌm.fə.tə.bəl/. If the vowel after the following consonant is stressed or if the schwa is followed by a cluster or by a word-final consonant then syncope is not permissible.
In many common three-syllable/four-syllable words, the second syllable is often dropped in casual/fast speech for the sake of ease. However, it might not be true for every accent/dialect.
See Michael Harvey's comments:
I say choc o late. Likewise nurs er y and ev er y. And av er age.