I am 42 years old. For as long as I can remember, I have called my mother אמא – íma – the generic Hebrew word for 'mom,' but since 1973 I have called my father by his first name, Ruvik.
In 1972, when I was 2 years old, my family moved temporarily from the Tel Aviv area to New York City, where my father began his doctoral studies at Columbia University. About a year and a half into our stay in New York, the October War began in the Middle East. For reasons that I will not delve into here, my father, as well as his now late cousin Elisha, and Noach, the man who would later marry my aunt Yovi (also deceased now), both of whom were also in New York in various capacities, all decided to volunteer for reserve duty in the Israeli army. They flew to Tel Aviv and joined their respective military units.
My family decided that I was too young to be told that אבא – ába – 'dad' was off to war. So I was just told that he went to Israel to do some work. But friends of the family, Yael and Haim Ben-Shahar, who were also temporary New Yorkers at the time, frequented our home with their children. Their youngest, Gili (sadly, also no longer alive), was about 8 years old, and was sort of my mentor. She explained to me that my father was not in Israel for "work," but rather for war, and that a consequence of war could be death.
Being the 3 year-old that I was, I took Gili's mentorship quite seriously, and interpreted "could be death" as "must be death." Thus, in my young mind, aba – 'dad' – was dead.
Six weeks later, when the same man who had departed for "work" / war returned alive and well, I had to make up a story to make sense of it all. It was easy: this newcomer was my late father's twin brother, who happened to have the same first name as my dad. Therefore, I would simply call him Ruvik, the nickname most people used to refer to my father, Reuven.
Shortly thereafter I did, of course, realize that this was, indeed, my father and not some bogus twin uncle. I had made several attempts at calling him aba, but it just never felt natural.
Interestingly, my brother, who was born three years later, also calls my father Ruvik (and my mother ima), but for a somewhat different reason. He simply had no role models at home who called Ruvik anything else but Ruvik. Or at least, that's how I interpret it.