I've bought a domain name that was PR3 then today is PR1 but is well ranked in Google and never got ranked when was PR3.
Also I've got new incoming links from high Pr websites that are related to my business (links that was done after I bought domain name)

My question is:
Why it is PR1 now? Is it because Google is updating PR so it will soon be PR3 or PR4?
or It's PR1 now and I must start SEO from zero to get a PR4+