Measuring in the store can be misleading due to the use of sizing to enhance the feel of the fabric. Washing in hot water will shrink the fabric slightly and drying will fluff the fabric a bit so the slip reading will be thicker after washing and drying and the compressed measurement will be thinner. You have to feel the fabric, look at the tightness of weave through the fabric in a strong light as you assess the measurement in the store. That's one of the reasons we can get acceptable performance using a wide variation of material thickness and tightness of weave. We have to be more [particular when we look for fabrics that can be used for X ring accuracy.