[[ y ]] [[ k.name ]] Major Big Event Small Event

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.