It doesn't work like that. The 32 bit disks should have 32 bit OS and the 64 bit should have a 64 bit OS.
Pretty much ALL CPU's nowadays are capable of running 64 bit. That doesn't mean they automatically use 64 bit, they are quite happy running 32 bit as well.
I would say the most likely answer, assuming that you definitely used the 32 bit disk, is either they were wrongly labelled, or you accidentally got 2 64 bit disks? Depending on how much RAM you have you may as well stick with 64 bit though, unless there is some driver issue. I have seen very few of them with Win 7 though.