This is a tricky question to answer because the answer depends on how you determined that it undercounted.
There are two ways to verify the accuracy:
1. Comparison with another pedometer
2. Manually count the steps yourself while walking
If you are comparing the step counting of two devices, you must first decide which unit is the benchmark. Expensive is not necessarily the best indicator of which is more accurate. You must manually test both step counters by counting the steps you take and comparing the results with the step counts.
Thus, just because our pedometer does not match your phone or other expensive counter does not automatically expose it as defective.
Our pedometer requires you to walk at least 7 seconds in order for your steps to be counted. If you walk less than 7 seconds, your steps will not be counted. If you walk more than 7 seconds, all of your steps, including the ones you walked during the first 7 seconds will be counted.
So if you do a lot of housework, where you start and stop frequently in less than 7 seconds, many of your steps will not be counted.
Our pedometer is designed for longer stretches of walking and not a few steps at a time where you stop and start every several seconds.
How you walk also makes a difference. Are you taking regular steps for a mile or two or walking around the house? The erratic irregular movements around the house are bound to produce different results in different devices because there is no one way to measure a step.
What constitutes a step? Shifting your weight from one foot to the other? Bending down to pick up something from the floor?
Because our body movements are infinitely variable, there is no one way to determine when we have taken a step. A foot shuffle might be counted as a step by one device but not another. If we are getting up, walking a few feet, washing our hands, going to the refrigerator and so on, the wide variety of body movements are not all clear distinct steps and each device will differ on how it makes that determination.
If you find that the unit undercounts your steps while walking a determined number of steps, move the unit to another spot and try again. The unit needs to detect vertical motion in order to count it as a step. Holding it in your hand and walking may actually insulate the pedometer from that vertical movement and hence, prevent it from counting your steps.