Probably wrong date on one of them, possibly both. It could be a transposing error, a transcription error, someone guessing, bad memory, and a host of other reasons. In some cases, a family member wanted to hide when they were born so they creatively added/subtracting a few years to their original birth year. I had an aunt who told everyone she was my Mom's younger sister. My Mom was born in 1930, but her sister was born in 1917.
Although there have been unverified reports of a 8 year old fathering a child, the only reliable thing I could find had the youngest father as 12 years old.
Best thing is try and independently verify both dates if possible. I realize that's not always an option.