The National Forest and National Monument Boundaries are incorrect. The Magic Mountain Wilderness, Pleasant View Ridge Wilderness, San Gabriel Wilderness and Sheep Mountain Wilderness are all within the San Gabriel Mountains National Monument (here shortened to SGMNM), which encompasses them all. The SGMNM in turn is almost completely within the Angeles National Forest, with a bit inside the San Bernardino National Forest at its easternmost part. See the SGMNM Fact Sheet published by the USFS: https://www.fs.usda.gov/sites/default/files/media/2014/41/san-gabriel-fact-sheet.pdf (can also be accessed via the USFS' SGMNM page). The map inside shows the boundaries of the SGMNM, and, if you zoom closely enough, the boundary of the Angeles National Forest/San Bernardino National Forest.