This is related to the iPhone and iPad. Every single device I came across has sequential bluetooth and wifi mac addresses. Except for one device and I think it might be jailbroken I am not sure (it was a friend's phone). Is this always correct? Every iPhone/iPad has sequential mac addresses except for the jailbroken ones? If not is there a pattern between bluetooth and wifi mac address?
I imagine this is the case: iPhone and iPad are using an integrated chipset (like this: http://www.broadcom.com/products/Wireless-LAN/802.11-Wireless-LAN-Solutions/BCM4334) supporting both Wifi and Bluetooth. And as both of them are on the same chip the manufacturer could be just incrementing the MACs.
However the question is why you ask this - if you want to use this information to check if it's the same device: don't, as there is no guarantee that it will always be this way.