API numbers and lease numbers are always a bit confusing, except when they are gas wells.
Gas wells are reported on an individual basis and oil wells are reported on a lease (number) basis. When you have an oil field, you might have 10 API numbers but in order to find the lease production, you must use the lease number inquiry function in the production query database. To do otherwise gives inconsistent (read as incorrect) results.
Therefore, a new lease number will be assigned when the (oil) well is completed in a new zone, but stays the same when it is a gas well. In both cases, the API number stays the same.
Does this answer fit your scenario? If not, I have no answer for you.