Lead time for changes is the elapsed time from the first commit to its deployment. It is the full lifecycle of a pull request.
Pull request cycle time is the elapsed time from the first commit until it is merged. It excludes the deployment phase.
While being related, it is useful to be able to track these two separately depending on your context. For example, you could have a very short pull request cycle time and a high lead time for changes. In this instance, it would mean that your deployment time is high and could point to different bottlenecks.
Using two different metrics does not impact the data behind, it is simply a different view. Pull Request cycle time can be calculated for all pull requests, whereas lead time for changes only consider pull requests that has been deployed in production.