I thought Sony said they were going to cut out the BS statements?
If the Xbox 360 is out longer, has had a much more steady level of purchasing over time and an older userbase then the index ratio would be expected to decline, especially for yearly games like Madden. People who are already satisfied with content already purchased aren't as likely to go out and buy more but conversely people who just bought a console are likely to be on the hunt for more software.
Publishers aren't looking at the number sold per console they are interested in how many sell overall. If the Xbox 360 still moves more copies then it still moves more copies, simple as that. Also if it fits into the overall porting mindset better with ease of portability with PC then thats also important. Again another reason why hes full of it.
Lastly its the overall revenue and the cost of that revenue which the publishers are looking at. Not only is the Xbox 360 cheaper to create for and gives the publishers more money in DLC and the option of digital distribution on both the Xbox 360 and PC makes the whole 'idea' of filling up a Blu Ray disc a 'bleh' proposition because it closes down these opportunities.
P.S. I know theres a lot of controversy here over the point, but it looks like Sony calls a game coming out on PS3/PC an exclusive in FFXIV. Quite ironic given the rules here to the contrary.