looks like they are looking for OSU money...I look at this less in terms of "odds" and more in terms of psychology...
The average bettor would MUCH MUCH rather have -3 than -3.5, because if the game lands on 3, you PUSH rather than LOSE
Not as important to Purdue bettors becuase if the game lands on 3, you WIN with 3.5 but PUSH with 3. Either way you don't lose.
I've found that most bettors focus on NOT LOSING rather than getting the correct odds, etc. This is the psychology of most sports gambling. They have no problem turning a win into a push (because they didn't "lose" any money in that they don't have to pay), but absolutely can't stand losing by that hook (which is why people buy the half much more than they should).
Just my theory, no evidence to back it up, but I say Pinnacle is leaning towards Purdue winning this one.