Why do dictionaries have three symbols for the same vowel?

And why do they select certain words to put each symbol in? I'm talking about the "ah" sound.

What vowel and symbols?

the "ah" sound?? That could be so many things. What language? Which vowels?
http://weston.ruter.net/projects/ipa-chart/view/keyboard/
select the symbols here.

English.

You probably have both the father-bother merger and the cot-caught merger, I also have those mergers. In some varieties of English there actually are three different sounds, though to people with those mergers those three sounds will sound pretty much the same.

That's interesting. Such a distinction with three different sounds seems alien to me.

Are you from the far West, North or South Dakota or Pennsylvania, or anywhere in Canada?

Here's a site with a map of where the cot-caught merger exists:

http://www.ling.upenn.edu/phono_atlas/maps/Map1.html

According to the map you have three distinct areas of merger in the United States. One in eastern New England, another that goes from western Pennsylvania down through most of West Virginia and southeastern Ohio to eastern Kentucky, and the third that covers nearly all of the western U.S.

According to that map, I'm in one of the transitional areas.

Florida is mostly cot/caught merged these days.
Central Indiana and Central and Western Ohio too.

This M W dictionary uses the same symbol for all a's: [ ɑ ]

http://www.learnersdictionary.com/search/call

Hong Kong
long
song
all
call
doll
dollar
collar
caller
fall
follow
bother
father
Don
Dawn
hawk/hock


they all have [ɑ ], like in American (South)West and St. John's NF, and unlike the rest of Canada and US EastCoast (where rounded [ɒ ] is preferred).

Of course, M-W uses General American.

No it fucking doesnt, you stupid cunt.

Of course it does. In fact, the full M-W dictionary shows alternative pronunciations too.