For most of the measurements Pixcavator uses only integer arithmetic, so there is no error. These are Area, Saliency, Contrast, Intensity (gray level). (Of course the resolution affects how well the digital image represents the reality and, therefore, the accuracy. See Continuous vs discrete.)
Some measurements aren't integers but come as ratios, in fact pairs, of integers. This is almost as good because the number is turned decimal only when it is displayed. So the error can be easily managed. The location of the Centroid (or the Center of mass) is a ratio of Moments. The Average contrast is saliency divided by the Area. Average intensity also falls into this category.
Lengths of curves cannot be computed exactly in the digital environment. With Pixcavator, the worst-case-scenario, theoretical error is 8% (more testing is needed). Therefore other characteristics also have this error: Perimeter, Roundness, Thickness and length.
Pixcavator does no employ floating point arithmetic. For what can happen when you round up floating point numbers, see this article The Patriot Missile Failure.