I am new to the ESRI js 4.15 API. I wanted to change my basemap from "topo" to a National Geographic Style Base Map as it is more fitting to my Application. When I use the following piece of code
ESRI has updated basemaps for a number of the traditional styles that are based on the new "vector tile" basemap technology (that may include a backdrop of hillshading as well). I think it likely that one of the maps is a "vector tile" version, and the other one a raster tile basemap.
These two variants, although looking somewhat similar, are not exact copies (either through choice by ESRI and limited restylying, but probably also largely due to inherent limitations on vector tile styling: some symbology and labeling options cannot be translated to vector tiles).