The list of en- words that can be pronounced /an/ rather than /ɪn/ or /ɛn/ is pretty short. From a quick search of the Carnegie-Mellon Pronouncing Dictionary, these words start with en- and can be pronounced /an/ (some have alternate pronunciations with /ɛn/):
ENCLAVE
ENCLAVES
ENCORE
ENCORES
ENFANT
ENSEMBLE
ENSEMBLES
ENTENDRE
ENTOURAGE
ENTOURAGES
ENTRE
ENTREE
ENTREES
ENTREPRENEUR
ENTREPRENEUR'S
ENTREPRENEURIAL
ENTREPRENEURS
ENTREPRENEURSHIP
ENVOY
ENVOYS
I will note also that Merriam-Webster does also countenance /an/ for envelope. However, few if any other en- words can be pronounced /an/.
According to A Critical Pronouncing Dictionary and Expositor of the English Language, 1839,
The adjectives naked, wicked, picked (pointed), booked, crooked, forked, tusked, tressed, and wretched, are not derived from verbs, and are therefore pronounced in two syllables. The same may be observed of scabbed, crabbed, chubbed, stubbed, shagged, snagged, ragged, scrubbed, dogged, rugged, scragged, hawked, jagged; to which we may add, the solemn pronunciation of stiff-necked; and these when formed into nouns with the addition of ness, preserved the ed in a distinct syllable, as wickedness, scabbedness, raggedness, &c.
This explanation is reaffirmed in the book, Teaching Pronunciation: A Reference for Teachers of English to Speakers of Other Languages, 1996,
Whether they function as verbs or adjectives, most English words ending in the -ed suffix follow the same phonological rules as the paste tense inflectional ending (e.g., striped /t/, forked /t/, cultured /d/, used /d/, moneyed /d/, furrowed /d/, good-natured /d/, gray-haired /d/, blue-eyed /d/). There are, however, historically based differences in pronunciation between certain formed ending in -ed, depending on whether they function as adjectives or verbs. The -ed adjectives in this category have an extra syllable and take the /ɪd/ pronunciation, whereas the verbs simply take /t/ or /d/, following the rules for the regular past tense and regular past participle outlined earlier:
[Table with examples comparing the pronunciation of verb forms of words such as blessed, beloved, learned, dogged, and legged, with their adjectival equivalents]
Sometimes, even when there is an adjective with no corresponding verb, the adjective is still pronounced /ɪd/ (naked, wretched, rugged, wicked).
Best Answer
For one thing, "two thousand" is shorter to say than "twenty hundred".
Basically when the number has three zeros it is shorter to say "thousand" than "hundred". Once there are fewer than three zeros it is shorter to say "Y thousand X" for Y00X and "Y hundred X" for YYXX.
This usage isn't limited to years either. Any numbers in the same range will have the same kinds of contractions. There is a Simpsons episode where this is played for laughs, when they need eighty-five-hundred dollars to fix their roof, and they only have $500, and Homer whines that they still need eighty-hundred.