Ahem- *cough* Halo CE, Halo 2, Halo 3, Halo Wars, Halo ODST, and Reach?
What jacked up line of events is that?
That's just the way they were made.
First you had Halo CE, Halo 2 and then Halo 3.
Those were the main storyline.
Halo Wars, ODST and Reach are just extra parts of the story.
They weren't crucial to the storyline and they were fit in afterwards. Same as StarWars started of with Episodes IV-VI and then later on the prequals of Episoes I-III were made.
Just because the story isn't in the correct order doesn't prevent them from being prequals and sequals.
Halo Reach is made after Halo CE, it's still a Prequal to Halo CE.