If it's offering upgrade straight away, then it's just because there have been releases since it was produced and sitting in a store so would update it. Generally with updates though, I'd wait a couple of weeks rather than update it straight away, this allows you to see if there are any major bugs that need to be fixed. For example, the last iPhone release of iOS 8 was a pig and needed a new update straight away. With android though, it's generally ok as the networks release it later when it's a stable platform and they've added their tweaks to it.