Hemoglobin levels vary during the day within the same person. Hgb levels are always higher during the morning and lower at evening. The difference between the highest and lowest levels during the day can be a slight difference but it can reach as much as 1 gram per deciliter. This has to do with differences in hydration levels and also with changes in elevation -- for those that commute between low and higher elevation areas.
I too am disappointed in the Ember, because I can get different numbers at the same time depending on which fingernail I use to take the measurement. However, if I stick to the ring finger, as recommended by the manufacturer, I get the most reliable reading -- meaning, the one closest to what I would get from the HemoCue analyzer or from laboratory results using venal blood. My conclusion is that the Ember is probably good for a quick assessment between results within the normal range and results way above or way below the normal range. I don't know for sure because I haven't experienced results way above or below the nomal range since purchasing the Ember. Just in case, I carry the Ember with me whenever I leave home for some other overnight destination.