Why have the same chip in the same package, but with a different number of pins?
There are few reasons why a manufacturer might do this. 

While it is fairly common to see multiple almost equivalent packaging options within the same datasheet. This manufacturer is being excessively pedantic by releasing two datasheets and two part numbers. 

Since that costs money (administrative costs) There is still possibly some difference that is captured there, even though, it is also possible (and arguably clearer!) to capture that difference in one datasheet with one table and multiple packaging codes/revisions. 




Evolution of packaging technology / Revision with contractual obligations


When the part was designed the LGA14 was more available or more easily manufacture-able, the reason doesn't matter.

Then the part is revised, or there is demand, or cost savings associated with the smaller package. 

However some customers will have contractual obligations that may make it annoying or costly to formally revise a part, so the manufacturer may choose to assign a new part number and continues to support both parts which are effectively the variants. 


Request from large customers


Large customers may request a custom part for whatever reason comes to mind, and if they order millions of parts the manufacturer will generally be happy to take their order. The manufacturer then may also offer this to the regular customers as well under a different part number.


Drop in upgrades


If there is an equivalent part from this manufacturer or a competing product from another manufacturer or for backwards compatability, then the manufacturer may provide an upgrade path with pin compatible drop in part, this may apply to either LGA14/LGA12 or both. 


Acquisition (related to 3)


This manufacturer has bought companies and has been merged/unmerged and has entered and dissolved multiple partnerships over the years. During this process a manufacturer may acquire equivalent parts from subsidiaries or partner companies. Often they may be differentiated by different part numbers. 

(In the most extreme version of this, I had a situation where a connector company bought another connector company and their production line that made an equivalent part , and as a result there were several part numbers for identical parts depending on which production line / lineage you were historically purchasing)