I have received this email from my university:
You must provide the investigating officer with any written explanation of the circumstances which have given rise to this allegation.
In writing your response, it would be helpful to bear in mind the following:
The Turnitin report is always analysed by at least one member of the academic staff before an investigation for academic misconduct begins.
The precise value of the Turnitin similarity index is of no particular importance. The nature of the alleged offence is as important as the extent of it.
The sources identified by Turnitin are not necessarily the sources you used simply because the same material may appear in many different sources. What is important is not whether you used those particular sources, but rather whether you have properly cited and referenced the sources you did use.
A very short explanation is just as likely to be effective as a very long explanation. There is no need to explain the matching text from each of the sources listed, whether Turnitin identifies only a few or an excessive long list of sources. Please note that any reference to specific sources identified by Turnitin should be based on the sources as listed in the Text-Only Report (rather than those shown in the Document Viewer).
You can access the Text-Only Report by pressing the tab key and then clicking on the drop down option 'Navigate to text-only Similarity Report').