It's entirely a matter of syllable stress. English is a stress-timed language. Only a stressed syllable can have a full vowel in English; unstressed syllables are centralized and reduced as much as possible, especially in rapid speech.
SERvice is stressed on the first syllable, so the vowel in the first syllable gets fully pronounced ['sɝ], while the second unstressed syllable is reduced to shwa [vəs].
On the hand, deVICE is stressed on the second syllable, so the vowel in that syllable gets fully pronounced ['vəys], while the first unstressed syllable is reduced to shwa [də].
Spelling has nothing to do with punctuation. English spelling was invented for a different language and doesn't work at all well for modern English. This fact explains a lot of other things, including why you shouldn't be worried if it's not logical. In fact, you're right. -- it's not logical.