There is no relation between HLTV.org and the creator of this tool. The data is gained from HLTV's stats section and gets updated after every MVP-worthy event.
The site includes all tournaments for which HLTV handed out an MVP award(so not only Big Events+) and an aggregation per year.
The ratings get calculated by creating a round-weighted average over all maps. Keep in mind the calculated ratings are probably slightly off, you can see that if you compare the overall ratings to the HLTV ones. A player whose map ratings gets more often rounded up - to present them with 2 decimals in HLTV's stats section - has a slightly inflated rating and vice versa.