Could be the custom profile did not take into account the shrinkage. Epson printers have been very notorious in coming out short on canvas prints. My Z3100 has been nearly spot on though. However I do allow an extra 1/4" in each direction so the edges wrap completely around. Also, instead of "mirrored" edges, I use intelligent fill most of the time, and touch up as necessary. This tends to give a smooth transition, so if things are off a bit, it's not noticeable.
And yes, water based sprays do result in some shrinkage, which is why I try and stretch within a couple of hours of spraying my canvas'. Also, this gives a nice tight result, that has yet to return for re-stretching. Others who have stretched my coated canvas' days or weeks later, have not been so lucky!