Is this normal?

I have a question for the community. This past August I had my kitchen and living room renovated and we put in five and a quarter Red oak floors that were stained and sealed by the installer. Over the past few months we kept hearing the sound of cracks in the other room but could never could quite locate where they where coming from. Be now noticed that our hardwood floor are developing gaps between the boards. We had the installer come by today and he said this is normal and it was really nothing that we can do about it but expect that the floor s will swell up again in the summertime when it's more humid. I've attached some pictures of the floor where I've stuck business cards in to show the level of where the gaps have appeared. Does anyone know in the professional community if this is normal for floors that were installed in August to develop gaps that were at least one business? If not, two Business cards wide?