Why was I offered an associate professorship after applying for an assistant professorship position? How will this be perceived?

UK perspective

(1) What do you make of this?

They think you are good enough to be recruited directly to the higher-ranking grade. It sounds reasonable in light of your seven years' postdoc experience plus having publications commensurate with the grade. They obviously think you are an outstanding candidate, and may be offering you the higher-ranking grade to make the prospect of international relocation as enticing as possible. Congratulations!

(2) How will this unusual feature of my future CV be perceived by universities in the west, if I later try to get back?

To be honest, I can never remember the difference between "associate professor" and "assistant professor" -- most UK universities do not use these terms and the whole concept of "tenure track" does not really exist in the UK (although some UK universities have very long probation periods for more junior staff dressed up as fancy schemes -- here is an example that involves up to 8 years' probation). A UK academic reading your CV would, once he/she has familiarised itself with the relative seniority of the grades, be impressed that you skipped straight to the more senior grade. In the UK, an academic who reaches the equivalent of "associate professor" level (probably "senior lecturer") within seven years of getting a PhD will have done very well, but it is not extraordinarily unusual. Definitely, it would work to your advantage.

In China academic positions are filled on the basis of publication record. If your total points (dependent on the impact factor of your published works) is higher than a particular number, you can directly be appointed to an associate or professor rank. I don't know if there's any document to back my claim, but I know this from my researcher friends from China. I know of one instance where a graduating PhD student was offered a full professor position because they had three extremely high impact factor publications, (two in science). Chinese academia is different than western counterparts.

Different countries have different systems, and job titles do not always translate well across borders. I suspect most people looking at your (future) CV will not read too much into the fact that your title is said to be 'Associate Professor'. They will judge you based on their perception of your achievements and stature, and mentally classify you within whichever system they are familiar with.