Birth dates are often wrong* on the 1939 register. Especially in transcriptions. It's always worth checking the original image. Ancestry's transcriptions are worse than FindMyPast's in my experience.
*I say wrong, what I mean is they often don't match with what we 'know' through other records like birth certificates. I know one of my relatives thought his birthday was a couple of days after it actually was. So that's the date he provided in the register.
Years are often wrong too.The older generation always liked to add a year on! My mum's birthday was in November,but come the new year she'd do the maths and take her birth year off the current year and tell evryone she was older

When my mother in law died in early 1997 (her birthday was in Aug) her sister in Ireland told everyone that she was 80......but she was in fact only 79......and 5 months.
There was a war on,people weren't THAT concerned about giving the authorities an exact DOB.
Back then, having to give your DOB on every occasion wasn't used so much as we have to today.
Carol