I'm not sure betting NBA is a good idea nowadays at all.
Statistics completely describe baseball and have for many years now. Thus, there is no way to make money against the books (unless you have some inside information about injuries), since they have the sport predicted as well as it possibly can.
Basketball is very close to that level. The books have always done a tremendous job setting the lines, and they're getting better and better each year, as better statistics and modeling keep being released. The one complicating factor is that while baseball is an individual sport at heart, basketball is a genuine team sport and modeling interactions between players is hard.
However, major progress has been made on that front, and the current style of play in the NBA (tons of 3-pointers) makes modeling it far more trivial.
Them setting the line at +13.5 only for it to be 14 isn't even remotely unusual.