Rectification is a correction that is made when a batch of essential oil smells different than the previous batches. This is often the case with natural products because of variations in growing conditions such as rain, sunshine, temperature, soil conditions, and harvesting conditions. You would not expect every single apple you eat to taste the same. Because we get used to a particular aroma, if it is coming out of a product in a bottle, we expect it to smell the same as the last bottle. When a bottle comes to us that smells different, we call customer service to get our money back as something must be wrong with this batch.
The truth is, each essential oil batch should smell different. Slight variations to even more pronounced variations should be common. Since the industry knows the general public will not like these variations, almost all companies practice rectification. Rectification is when they add in molecules from another species or from a synthetic in small amounts to make it match not only the gas chromatography mass spectrometry report, but also to match the aroma from the previous batches for consistency. These additions are impossible to detect on a certificate of analysis report and even the most trained eye can miss the addition of synthetics or other species present in the batch. It is why organoleptic testing is a very important part of the batch test process.