There may be a situation where you get to the last page of the installer and you don't see your version listed as an upgradeable path (ex. you have 1.2.5.4, but you only see upgrades for 1.2.5.3, and 1.2.5.7). In these cases, it means that between 1.2.5.3 and 1.2.5.6 there were no other database changes. In this case, the closest upgrade to click on would be 1.2.5.7.
Another example: If you are running 1.2.5.4 and 1.2.5.5 has just been released. The upgrader is only showing an upgrade link for 1.2.5.3. This means that no DB changes have been made since 1.2.5.3 so your upgrade would be code-only.
In simple terms, if you seen an upgrade for 1.2.5.3, and you are running 1.2.5.4, then you already have the database updates needed for 1.2.5.3 so you would then choose the upgrade above your version.