Litvak date of birth discrepancies

[pergamond]

I wonder if anyone has any experience of this - I've been researching a Lithuanian family who emigrated to South Africa, and found their births on Litvaksig. I am sure it is the correct family. However their dates of birth were different from those the family know - correct year, but day and month different. So, to check the transcribing, I sent for and received copies of the original registers from Lithuania. These were written in cyrillic Russian and also in Yiddish. I put them on Jewishgen in Viewmate, and within about two hours they were translated for me. Same issue - date discrepancies. Any ideas as to why this may be so?