Pattern Matching Engine is designed to create uninterrupted stripes while generated images are tiled vertically, horizontally or in both directions. Matching control evaluates stripes geometry based on selected width and spacing as well as specified image height/width and then optimizes width and height for the best possible stripes matching.
For this reason, when you enable stripes and select image Size and Alternative Image Side, the resulted Effective Image Tile Size might be different from what you have specified in the most of the cases.
So it is best to keep the Pattern Matching Engine running (ON) at all times, when you create striped images intended for tiling - repeated within destination background layer.
If, however, generated image is not intended for tiling (tabs, buttons) or there is no need for stripes matching, the Pattern Matching Engine could be turned OFF. Turning engine OFF would keep image dimensions strict according to your specification.
Important: Turning engine OFF is only recommended for Gradient types other than Line.