Reading the constitutions of both states in order to pin down exactly where the problem is, the Wikipedia article is sort of vague. The Nevada document specifically references "thence in a North Westerly direction along the Eastern boundary line of the State of California to the forty-third degree of Longitude West from Washington" So that is not the problem, the border is whatever California says it is. It must be a north-south strip in the difference between "the forty-third degree of Longitude West from Washington" and "Running south on the line of said 120th degree of West Longitude"

The latest California constitution just says to reference the original 1849 for the boundaries https://archives.cdn.sos.ca.gov/pdf/1849-california-constitu...

Nevada https://www.leg.state.nv.us/Const/NVConst.html#Art14 amended in 1982, but the original 1864 document has the same geographic description.

The funny thing I found in the Nevada constitution was

"All territory lying West of and adjoining the boundary line herein prescribed, which the State of California may relinquish to the Territory or State of Nevada, shall thereupon be embraced within and constitute a part of this State."

We reluctantly agree to your existing borders, but we really wanted the Sierra Nevada ridgeline to be the border and if you want to give it to us we are going to put it right in the constitution to accept it.

Wait, so it was actually believed that the Washington meridian just happened to fall exactly on an integer number of degrees (120-43 =77) west of Greenwich?