Your understanding of CI splits is incorrect. Once the split is done, you have records 1 through 5 in CI 1 and records 6 through 10 in CI 2. The key for record 11 will determine whether it is placed in CI 1 or CI 2 -- if the key value is less than the key value for record 6, the new record will be placed in CI 1.
As records are inserted with keys less than that of record 6, the unused space in CI 1 will be used for the new records. A CI or CA split creates freespace that can be used for data records. The only way CI 1 would have the first 10 records, CI 2 the next 10, and so forth would be if you unloaded the VSAM file, deleted / defined it, and then reloaded it with FREESPACE(0 0).
You need to download and read this Redbook: http://www.redbooks.ibm.com/abstracts/sg246105.html?Open
which is the VSAM Demystified
Redbook. All of this and more is explained there.