The short answer is not to worry too much about ketone measurements, particularly if you are getting results.
If your diet contains no more than 20 grams of carbohydrate and you are still breathing in and out, then the logical conclusion is that you are in ketosis, regardless of what your measurements are. The except may be that you and your wife are too insulin-resistant, in which case lowering your carb limit or eliminating carbohydrate altogether should do the trick.
Measuring ketone bodies is not an entirely accurate indication of how much we are producing and using. There are three ketone bodies, acetoacetate, acetone, and β-hydroxybutyrate (the first two are ketones, but the latter is technically not; this is why people often use the term “ketone bodies”). The ketone bodies can be measured in three ways: in the urine, in the breath, and in the blood. For some reason, the urine strips measure acetoacetate, the breath meters measure acetone, and the blood monitors measure β-hydroxybutyrate, but all three are measurable in all locations.
The problem with measuring acetoacetate in the urine is that these are wasted, and after a period of adaptation, a lot of people find that their kidneys get really good at cycling ketone bodies back into the bloodstream, so ketone production appears to drop. This is why a lot of people will tell you that urine strips are worthless. They are also not particularly accurate, having been designed merely to give Type I diabetics an early indication that they might be heading towards diabetic ketoacidosis, a dangerous and potentially fatal condition. Hence there is no need for strict accuracy; just enough to give a diabetic time to get to hospital and be treated.
The problem with measuring acetone in the breath is that most breath analysers are designed to detect the by-products of alcohol consumption (acetone is one of them, which is why you can use a breathalyser to detect ketones) and are not generally designed specifically to detect acetone all that accurately.
The problem with measuring β-hydroxybutyrate in the bloodstream is simply that what we can measure is the circulating β-hydroxybutyrate; we have no way of measuring how much is actually being produced and how much the cells of our body are actually consuming.
So use these measurements as a general guide. If you are losing fat and your clothes are getting looser, then you are doing keto right.