I think he may mean why has he got the reforms in 146 (or anytime after 172), when (at least according to http://en.wikipedia.org/wiki/Marian_reforms, I know I know but it's close enough for this purpose :p) the reforms historically could have been put to a date around 106-107bc.
Now I'm not a team member nor a historical expert so this is either speculative or just what I've picked up from around these forums but I would imagine the main reason for this would have because there are other conditions to the reforms.
The Marians historically came about because of the changing nature of the Roman Republic in regards to its size and the number of men available to recruit, the EB team have never been about railroading the player down a particular route, and therefore I think the reasoning is that if the republic had reached the size and conditions (as well as can be simulated in the engine, ie building latufinda etc) earlier in history, it's possible they could have made reforms earlier.
Plus 106bc is one hell of a lot of turns for the average player
Hope that helps
Cheers