It can happen that some ancestry present in your ancestors is not reflected in your results. This is especially observed when said ancestor dates back several generations. The populations of these ancestors dilute over time, with only the most predominant and recent ones in your family tree being maintained.
Furthermore, the genetic composition of different populations can vary over time due to historical events, migratory flows, and internal dynamics. Therefore, the genetic context of your ancestors' population might not exactly match the current genetic context of that same population. An ancestor from an ethnic region may have different genetic markers than the individuals used as a reference for that population.