I'm native Swede. I have noticed that sound-qualities differ from language to language, and I myself have difficulty hearing distinctions between vowels in e.g. German, that is a bit different from Swedish, etc. It is much easier to hear one's own native language, however small the difference is.
It is the A-sound that differs. In 'glas' the A is long and pronounced more in the back, your mouth is more open and has almost round lips, but open (not like 'u'), the short A in 'glass' quickly moves forward, to the 'ss', with lips more narrow/straight horisontally ... (hard to describe)