The story of western Virginia is complex, and the map itself disguises this somewhat. Areas with relatively small slave populations generally sided against secession when the vote was taken on April 4, when it failed, then again on April 17, when it succeeded. However, this opposition to secession did not necessarily translate into abolition, or even resistance to slavery. When West Virginia finally became a state in June 1863, it passed rigid manumission laws that would have protected slavery for decades were it not for the 13th Amendment. And in several counties the opposition to secession constituted a slim rather than a decisive majority; in other words, the map is not a transparent reflection of political sentiment. Yet it captured the imagination of the nation’s leaders and the public, and it circulated widely through Washington.
By August, the Army had secured the western counties to the point that the residents could begin to organize themselves into a new state. At this moment of victory, the Coast Survey issued a second edition of the map that boldly colored the proposed state of “Kanawha” in western Virginia. This edition also reinforced the initial message of the map with statistics showing that the vast majority of slaves had remained in Virginia.
Slaves made up 31.1% of pre-division Virginia, while post-division 37.2% of Virigina's population consisted of slaves, while 2.5% of West Virginia's population did. The geographic cline in slave ownership from the hills to the coast in Viriginia was largely a product of the viability of plantation style agriculture in different parts of the state.
It isn't entirely clear to me why McDowell, Buchanan, Wise and Pendelton Counties in Viriginia, which were contiguous to West Virginia and also had few slaves did not join their neighbors in leaving the state of Virginia.