This is just a conjecture, I'm not informed enough on English to know what's up, but.

I think the pronunciation of a language, as taught to non-native speakers, is usually biased towards more conservative variants; specially when there's a lot of variation going on. In this case towards realising /æ/ as [æ] instead of [ɛə], [ej], and others.

For reference, I've seen something similar popping up with German unstressed -er (eg. kleiner, Kinder, unser, etc.); most native speakers would realise it as [ɐ], and yet when I learned it it was "supposed" to be [əɐ̯].