YouTube's Content ID system, which is designed to detect copyright infringements, operates based on audio and visual recognition technology. The system matches uploaded videos with a database of content submitted by copyright owners, and flags any matches it finds.
Despite the use of free and royalty-free music, a video may still be claimed if it contains elements that are protected by copyright. For instance, if a video features a copyrighted image, sound effect or animation, it may trigger a claim from the copyright owner. Additionally, it's possible for the music to be part of a larger copyrighted work, such as a movie soundtrack, even if the individual song is available for free use.
Moreover, while the video owner may have the rights to use the music, they might not have the necessary licenses to monetize the video. In this case, the copyright owner may claim the video to collect ad revenue on behalf of the copyright holder.
It is imperative to ensure that all elements used in a video, including music, are properly licensed and not infringing on any third-party rights. The YouTube Creator Studio provides a "Copyright & policies" section that lists copyright laws and guidelines, as well as a way to dispute claims if a video is flagged.
In conclusion, the reason why a video may be claimed despite the use of free music can be attributed to several factors such as the presence of copyrighted elements in the video, lack of necessary licenses to monetize the video, or the music being part of a larger copyrighted work. Therefore, it is advisable to ensure that all elements used in a video are properly licensed to avoid potential copyright issues.
Source: YouTube Creator Studio Help (https://support.google.com/youtube/answer/2797468)