I think what they're saying in linguistic terms is that what we would distinguish as eh and ay are allophones:
http://en.wikipedia.org/wiki/Allophone
"Speakers of a particular language perceive a phoneme as a distinctive sound in that language. An allophone is not distinctive, but rather a variant of a phoneme; changing the allophone won't change the meaning of a word, but the result may sound non-native, or be unintelligible"
So if we use another example, the R and L thing. You might get your flap right and sound pretty native, or you might just say it with an English R or L like ARIGATO or ALIGATO and one of several things might happen: you'll be understood but you'll sound really foreign, or they might understand you from a combination of context and sound, or they might be like what the heck did you just say?
This page is pretty cool (ignoring the pink background that is)
http://www.compulink.co.uk/~morven/lang/vowels.html
It lists languages with 3, 4, ..., 14 vowels in their "vowel system." Check out the 5 vowel system.
Quote:Latin and some dialects of Classical Greek immortalised this, and it in turn dictated the number of "vowel letters" in the Roman and Cyrillic alphabets. Spanish, Serbo-Croat, Hebrew, Japanese, Swahili, Maori, Hausa and Basque, to name but a few, are modern-day languages with CL. (In Japanese, /u/ is typically unrounded /w/, but this doesn't really matter here.)
I can't quite figure out where English fits in there since he has it divided up into "Australian English" "Scottish English" and so on.
This page says typical American English has about 15 vowels
http://faculty.washington.edu/dillon/Pho...start.html
So, no doubt what we would consider to be separate vowels would be allophones in Japanese.