What do they mean when they say Spanish vowels always sound the same?

One thing has me confused.  I was told all Spanish vowels are always pronounced the same but there are some very common small words that don't seem to follow that rule.

For instance usted.  According to the rule it would be pronounced like oo-stayd, but I always hear it pronounced oo-sted (rhymes with head).

 Or él.  According to the rule it should be pronounced like ale (rhymes with bale).  I  hear it pronounced el (rhymes with dell).

As Sanz says, the pronunciation of vowels depends on the pronunciation that each speaker has. I would say that approximately the vowel sounds in spanish must sound like this if we compare it to english sounds:

a = as in "cut"

e = as in "hair"

i = as in "see"

o = as in "pot"

u = as in "blue"

I think that should be

e = as in pet

The sound in hair is a dipthong which doesn't exist in Spanish.

Dipthongs which do exist are in Spanish are (again just approximations):

ai = as in side

au = as in sound

ei = as in may

eu doesn't exist in English, but it's (almost) the sound you get when you say may-you, without the "y" sound.

oi = as in boy

Saludos