You're printing on canvas---which has more pliability than paper.
You're experiencing the George Costanza Effect (Seinfeld) ("It was the pool!") otherwise known as shrinkage
. Normally, this is most noticeable after hvlp spraying or rolling the protectant over the canvas. You'll notice that like a pair of jeans, shrinkage typically occurs lengthwise.
On your 9900, make sure to designate "canvas" under fine art paper for media type on the Printer. Also check "canvas" in the printer settings on the computer. This seems to address some if not most shrinkage issues on canvas. (There is variability according to the types of canvas used; variability in shrinkage also seems to be different from printer to printer
, even if the same model). There are also other tweaks that can adjust for shrinkage by percentage in photoshop. Programs like Qimage can adjust for canvas length shrinkage by a certain percentage, which is really neat, because once you determine what the approximate shrinkage is for your printer
, you can simply dial in the percentage of shrinkage, and get really really close to your desired print size.