Why did Google jump from version 12.0.1 to 15.0.0 with Google APIs for Android?

This is possibly due to culture specific superstition, 13 being considered an unlucky number in western cultures, and (1)4 in eastern cultures.

Related:
Software versioning superstition (wikipedia)
Is v15 of Play Services a late April Fools joke? (reddit)
Play Services 15.0.0. Why the huge jump in version number? (reddit)

Special care may be taken to avoid occurrences or reminders of the number 4 during everyday life, especially during festive holidays, or when a family member is ill. Mentioning the number 4 around a sick relative is strongly avoided. Giving four of something is strongly discouraged. Elevators in Asia and Asian neighborhoods often skip the 4th floor or any floor whose number contains the digit "4" (as 14, 24, etc.). Military aircraft and ships also avoid the number 4 (such as the South Korean and Taiwanese navies) due to its extreme negative connotations of death. April 4 is also considered an exceptionally unlucky day (much like Friday the 13th in the West).

_{From Tetraphobia}