First of all, it's important to note that there is no standard in how the registries stores or expose the information.
Most ICANN-regulated TLDs follow the same rules, but generally speaking each registry can store and manipulate most of the registration information independently than other registries.
That said, the "registered on" generally represents the initial registration of a domain name, regardless the ownership.
Let's say A registers the domain on Jan 2006. On Mar 2010 the domain is transferred to B. The registration date will still be Jan 2006.
The registration date will not change if the domain keeps being renewed, it is transferred to another owner or to another registrar.
However, there is also a second scenario: the domain is registered on Jan 2006, then at some point the domain is not renewed and it expires (let's say on Dec 2010). On May 2011 the domain is then re-registered (it doesn't really matter by whom). At this point, the registration date will be May 2011.
In other words, the registration date reflects the latest registration, it doesn't reflect the date the domain was registered the first time (if it was registered and then expired).