The following paragraphs describe a chart.
Although there was no change in the number of people aged 65-74 between 1960 and 1970, the number of people aged 75 and over had increased during this time. By the year 200, 12.% of the US population had reached the age of 65 or more, although this was slightly lower than in 1990 when it had peaked at 12.6%
The chart shows that today people in the United States can expect to live longer than in 1900. By the year 200 more than 12% of the population had managed to live to the age of 65 and over compared to only 4.1% in 1900.
I have some doubts with past perfect that are used in this two paragraph.
By definition, When we are talking about past and want to mention something that happened earlier we use past perfect tense.
This rule seems not applicable for the above example. Then why we use past pee]rfect without using simple past tense?