HELP ! ... Keyword Query That's Driving Me Nuts !

Hi all,

Sorry if this is a no brainer to you, but it's been driving me crazy !

I've been learning how to find keywords or phrases which have a
good number of searches with low competition, but I'm having
trouble getting my head round one point.

I'm gonna use the phrase - quick weight loss diet as an example.
(I'm not actually using this, it's just an example for this query)

When I do a search on google using this phrase, the competition
number is 15,400,000.

When I do a search using quotes "quick weight loss diet" to find
out how much competition there is for this exact phrase, the
number is 147,000. This is the number of sites which contain that
exact phrase, so this must be my real competition, right ?

Now, the thing I can't get my head round is this....

If a potential buyer does a search on google using the phrase
quick weight loss diet, then they will see 15,400,000 other
other competitors. I know that there is only 147,000 using this
exact phrase, but most people searching the internet won't
be using quotes and actually typing in "quick weight loss diet"
will they ?

People searching on google will just type in the phrase without
quotes so 15,400,000 other sites will appear.

Does this make sense ? Can someone please explain this to me
before I go mad.

Thanks for reading my post

John