Annv. Omission of T question

Unit 22 says the T is omitted at the end of a word.  And after the S.

So COST does not have the T

However, it says T is written in the following words.

And COAST has the T.

Why?

Is it just a variation, the vowel sounds or sounds in general, to know the difference between when transcribing??? or what? 

confused shorthand writer here…

Thanks

Debbi