transaction ID wrap limit

Hi all,

A vacuum full command logs the message:
... LOG:  transaction ID wrap limit is 1073822617, limited by database "A"

Sometimes ago, the vacuum full logged:
... LOG:  transaction ID wrap limit is 2147484148, limited by database "A"

What causes that difference of the limit ? Should I set or optimize
something on my Postgresql server ?

TIA,
Sabin