The purpose and rational seem obvious. CA wants the game to cover a VERY long period of time, but they don't want the game to take 'too long' and become tedious. CA wants to include major historical events, probably as triggers like the Marian event in RTW, but in order to keep the action going, they feel the need to blow through many years where they feel nothing interesting happened.

So, if turn 1 = 1066, turn 100 =/= 1166. Turn 100 could = 1280. They probably designed it like this:

Q1) How long a time period do we want to cover?
A1) 1050 - 1600, so like 550 years

Q2) But how many turns do we want?
A2) Approx. 225

Q3) But if 1 turn =/= 1 year, how will we mark the passage of time?
A3) We will use major historic milestones, and sprinkle them throughout the 225 turns. They will occur in historic order, but obviously not with the historic amount of turns/years between them.

Basically, CA is treating the medieval and renesance periods like it was recorded on a TIVO. The boring parts are treated like commercials and are skipped over. That way, the 4-hour Super Bowl game is boiled down to a 2-hour high-light film, plus the Rolling Stones half-time show of course.