Well, is this a normal birthday gift from your parents or did they send you the gift knowing that your DH's parents didn't send a gift? Do the sets of parents have a history of being competitive against each other?
I would say don't take it too seriously and enjoy a nice dinner out with DH.
Edit: Oops, I misread the post. I would say that since you turned the big 4-0, that is really why they sent the check, since it can be a bit of a milestone birthday!