I think this list is a bit arbitrary. The reason for this is that prior to 1990 all tournaments below Grand Slam and above the level which is now 250 were called Super Series. In that list I think they just picked the 9 richest Super Series each year and designated them as equivalent to todays` Masters when such dictinction never actually existed at the time. I have most of the World of Tennis [edited by John Barrett] yearbooks from 1970-2001 and there were no such distinctions made. What is now 500/1000 were then Super Series. Additionally you had the at times independent WCT tour.
This means that Lendls total is not quite as impressive because the top players were spread out more ie. you often had 2 super series events per week.