Is a 16GB iPhone supposed to only have 11GBs available?
You can check on Apple's storage capacity guide here, but in short may be caused by two factors:


The system reserves a portion of your storage for itself, system files, system partitions, buffer, metadata, etc. Regarding this, I am not entirely sure how much capacity this stuff uses, but you cannot delete it; iOS needs it to run.
When they say that the phone includes a 16GB flash drive, they mean 16000000000 bytes (decimal system), which is about 14,89 GB in binary system (1024*1024*1024=1GB). As the guide says:



  The way decimal and binary numeral systems measure a GB is what causes a 32 GB storage device to appear as approximately 28 GB when detailed by its operating system, even though the storage device still has 32 billion bytes (not 28 billion bytes), as reported.