How? Why?Where?

Everything has got a bar code these days.
Are they totally unique and if you took the jar of jam to a Ford dealer and they scanned it would it come up as a jar of jam or a spark plug for a 1995 Focus?
If it is unique who keeps the database?
Who decides the number for a new product?
Why does Sainsbury's Taste the Difference lightly smoked salmon only have 7 digits and the Blackcurrant jam has 13?
Does anyone else wonder about this?
Does it matter.......?
Should I get help about this worry?