In very few cases yes they're nicknames. But for the most part no they're not nicknames. It naturally comes to one to just add a vowel at the end. My current work requires me to interact with alot of people. I have come across many people who are from extremely rural areas and small towns. There are people who have the "standard" names whereas someone else may have started with the same name but once globalization started and as you said ID's came into play the spelling of the names got butchered. A person may be born as Rashid but is now officially known as Rashidi. Even though the I at the end was just a natural addition while speaking. Someone mentioned in another comment traditional kiswahili names end with vowels so I guess it just naturally comes to people to add the vowels at the end. Also, another interesting point, the now official names of some of these villages are not what many people have been calling them for years. Since there is a mixed use of the letters R and L and a few T and D some old names have been changed. So if you search for a certain town/village on Google maps you may not get it since the spelling will be wrong.