Dutch Donderdag - Congratulations! It's NOT your birthday!

I was reminded the other day of an odd (for me) Dutch custom, thanks to it being my boyfriend's birthday. We had a little soiree & as the guests arrived they would, of course, congratulate him on it being his birthday. The odd part is that they would then congratulate me... on it being his birthday.

Why?

Is there some subtle, underlying context to that congratulations that I'm not getting? “Congratulations... for not offing him & allowing him to reach this age!” “Congratulations... for putting up with him for another year & not running back, screaming, to the UK!”

If I was his mother I could understand the random congratulations “Congratulations... for birthing him & for him reaching a ripe old age”. But as a girlfriend, well it just seems a little weird.

Mind you, there's a lot about the Dutch I find weird.