I think we're talking about independent observations. I found it remarkable that Google's papers received 60583-19354=41229 citations between (presumably) 1998

I think we're talking about independent observations. I found it remarkable that Google's papers received 60583-19354=41229 citations between (presumably) 1998 and 2000 but only 19354 between 2000 and 2010. That's a pretty staggering difference, especially as the number is an aggregate, as you say. Based on this observation I theorise that the PageRank papers were hugely influential, but the recent published work has paled in comparison. (i.e. those 19354 will also include later citations of the early papers, inflating the 41229 figure further)