Altogether 35 World records have been reported since 1922. 8 of these were set at Olympic Games (especially those in the early years) - the last one at the 1984 Olympics in Los Angeles.
It is difficult to compare scores from the 1920s and today's World record, because the points table were changed several times during the century. New tables were introduced in 1920, 1934, 1950, 1964 and 1985. Here you can see a list of all Decathlon's World records.
Šebrle's 9026 points
Before May 27th in 2001 Roman Šebrle had a personal best of 8757 points and had won the silver medal at the olympics the previous year. But he was usually in the shadow of his great fellow countryman Tomaš Dvorak, who was reigning World Champion and world record holder with 8994 points, thus having narrowly missed the magic 9000 points mark.
So nobody really expected Šebrle to score a personal best in five events and becoming the first athlete ever to score more than 9000 points. Everyone actually believed Dvorak would try to attack his own record. It was a really close race, but even after a great first day nobody talked about the World record, although Šebrle's score was 30 points ahead of Dvorak's at his World record.
And day 2 did not start too well with Šebrle losing points against Dvorak in almost every event. And even after a great javelin throw of over 70 metres he was still behind by 68 points.
But Dvorak's 1500m had not been too fast. And Šebrle, knowing he had to run faster than 4:26, got help from one of the best 1500m runners among the decathletes, Attila Zsivoczky. Probably without that help the attempt would have failed.
Eaton’s 9039 points
So maybe you wonder about Ashton Eaton’s progression during his World record (9039 points) in 2012. Here’s a comparison:
Ashton started with a huge advantage: 102 points ahead of the previous world record, building it up to a 133 points after the long jump. Then reducing his advantage in the shot put (to 64 points +) to get 1 point behind in the high jump. Ashton Eaton - the better 400 meter runner – then fought back with 53 points over Roman Šebrle ‘s first day pb. From here on they took each turn the lead.
Ashton after the hurdles (+82), Roman the discus (-23 for Eaton), Ashton with 132 points over Roman far better in the pole vault (Roman had no good pole vault trainer at that time) to give his lead away in the javelin (-39). But already after that 5.30 in the pole vault, a small conversation appeared between Ashton’s trainer Harry Mara and the new world record holder in the making.
Ashton: “The American record?” Harry: “No ……. !!! We go for the World record Ash!!!” At the end Eaton ran a time sprinters don’t run – 4:14.48 meaning 7.50 seconds or 52 points faster than Roman’s time. 13 Points better than the preceding world record, establishing the new standard for those called “King of athletics”.
See also: Decathlon’s World Record progression