The NVD has historically associated CWEs based on a subset of the entire CWE library when performing analysis efforts. Originally this was a small list of 19 CWEs and was updated to include a much broader selection of CWEs in July of 2016. The NVD and the MITRE CWE team have been collaborating to update the CWE slice used by NVD analysts based on information over the last 3 years. This "slice" can be viewed at https://nvd.nist.gov/vuln/categories
Additionally, as part of this slice update the MITRE CWE team has provided additional retroactive analysis on CVEs from recent past to align CWE associations with the new NVD CWE slice.