When native English speakers speak quickly, sometimes a vowel or syllable is not pronounced even if it is in the word. This is done so that the word sounds more fluid or flowing. Both pronunciations are correct but the shorter version is more common in daily conversations. There are not so many of these words so learning this list a very beneficial. There are variations of these pronunciations depending on accent and region.
Listen to both ways of saying the same word. First the full pronunciation and then the second and shorter form. See if you can spot the difference!