There's probably nothing malicious about most cases of missing or strangely uniform birth dates. In many parts of the world over a century ago, people simply didn't bother with accurate records.

For example, both my grandpa and grandma had two birthdays each, in two different calendar systems, that pointed to wildly different points in time. Nobody remembered exactly when they were born. My other grandma was recorded as being four years older than she thought she actually was, and nobody knows the truth, either. My father's birth was filed with the authorities several years late, though the document itself pointed to the correct date. My family's not from some sort of jungle, either. All of this happened in a highly bureaucratized, highly literate society.

Go back a few more decades and one could easily imagine "She was born in the spring, in the year of the great flood" becoming "Let's just say she was born on April 1, and when was the flood? I mean the second one after Steve became king" when modern record-keepers demand a specific date. We're trying to see more precision in the data than anyone ever intended to record. No wonder we find artifacts.