Something to keep in mind regarding "completeness": only planes equipped with ADS-B transponders will generally be picked up by this (I say generally because I'm not sure if they are trying any multilateration techniques here on the Mode S). Prevalence of ADS-B is OK at best and very dependent on geographical area and type of aircraft (general aviation, commercial domestic, commercial international, etc).
For alt least some of their data, they rely on radio amateurs picking up transponder data and uploading it. They probably don't have a good network of stations in those regions.
Probably the time of day, I notice many more planes in Europe.
I wonder it live or delayed. Its 10.30 in India and past noon in China.