yeah, i do think it's a little odd. and like you, i'd be mildly bummed at losing the opportunity you and the other couple to get to know each other more directly and personally. the dynamic is completely different between four and six.
not a huge deal, though. when you know her better you'll have a better feel for her motivation. could be nervousness (more people, less pressure to converse), or cluelessness, or warm inclusiveness, or any number of other things.
but i do think that since they don't know you very well, and barely know the other couple at all, it's just a bit.......odd.
if you're comfortable with it, i'd just say something along the lines of 'i think you'll really like them! maybe we can have a cookout next month and all get together. i was kinda hoping to just hang out with you guys this time, though. what do you think?'
if that feels too overbearing, just go along with it, enjoy the evening, and observe the energy flow. if you and your husband still feel inclined, get an in-house sitter and invite them over to your place (they won't feel free to invite anyone else to that) and see if you want to pursue a friendship. if not, you had a fun evening out!
:) khairete
S.