The x96 number of course ties to the reference 96 calatrava - so the x96 watches have a difference numbering convention. 5196 5296 5396 5496 are more hierarchical than based on newness.
The reason 5196 is still in the collection and the 5296 is not, is that the 5296 is more of a bread and butter model that is going to be updated sooner than the 5196 will be. The 5196 has an older movement than basically any of the full rotor automatic calatravas have ever had (other than vintage.)
In my ~10 year history here i have hoped for a larger manual wind calatrava movement, but it clearly seems that the automatic/date movement is much more important to Patek (based on volume i would presume!)