I have to admit it does sound odd to invite one partner but not the other, but then there could be a reason and unless you ask the Bride or Groom outright, you're not going to know the reason.
But I don't agree with the comment about it not being because of money because they are having an evening do, just because they are having a party, doesn't mean they aren't watching money! I mean one extra person at dinner could mean an extra of anything up to or over �50 on the day cost.
When my Husband and I got married, we had just the immediate family at the meal, and then a huge party in the evening for everyone, precisely becuase we were watching our money.
Be the bigger person, don't say anymore, go along to the evening do and just enjoy yourself.