"does not quite add up." What does this phrase mean?

[petisha]

I've just watched "Friends" 16 episode 5th season and there was the phrase "Something didn't quite add up there." I can't understand what does this phrase mean.

 

[Tdol]

It doesn't make sense, it's unconvincing, unlikely to be true...

 

[petisha]

It doesn't make sense, it's unconvincing, unlikely to be true...

Thank you!

 

[ehsaning]

I think it means: failing to make you believe that something is true or real