Probably allophones. Just compare the ‘o’ in Québécois “comme” and “alors” and “coquin” etc. And there are some languages that have a lot of vowel phonemes (German and English are two good examples).
Québécois is notorious for being a very rapidly spoken language. For instance, one can express the phrase “I am at John’s house” in 3 syllables “Chu chez Jean” compared with, say Spanish “Yo estoy en la casa de Juan” which has 9 (although most Spanish speakers will omit the initial “yo”). It is because of this efficiency that I like to speak Québécois.
I am highly skeptical of this alleged efficiency. Though I don’t have a cite at the moment, I distinctly recall from my undergrad linguistics classes that spoken language conveys, on average, a certain number of ideas per second. The difference between the averages for any two languages is negligible – that is, there is very little deviation from a universal mean. Thus, while you may slur out “Chu chez Jean” in time t, a native Spanish speaker will fire off his corresponding nine-syllable monstrosity in time t plus or minus epsilon. It may seem rapidly-spoken to us, but for the Spanish speaker it is his normal pace.