There is a big difference between an update and an upgrade. Actually, your MacBook searched for updates, not upgrades as you say above.
The difference is this:
Updates are free of charge. Upgrades cost money.
When advancing from Lion to Mountain Lion (OS X 10.7 to OS X 10.8), a fee is involved. Therefore, this would be an upgrade. When you search for updates in Lion, there are none. But there are upgrades available.
Mavericks (OS X 10.9) is somewhat different, becuase it is also an upgrade (from Lion or Mountain Lion). However, Apple is offering the upgrade to Mavericks without charge.
So if your machine is eligible (see below), you might consider moving up.
http://support.apple.com/kb/HT5842