Revisiting polymer surface adsorption with a level of quantification not possible at the time of earlier seminal contributions to this field, we employ fluorescence microscopy to quantify the in-plane diffusion of end-labeled polystyrene adsorbed onto quartz and mica from cyclohexane solution, mostly at 25 degrees C. Care is taken to prohibit a surface-hopping mechanism, and the experimental techniques are adapted to measurements that persist for up to a few days. The main conclusion is that we fail to observe a single Fickian diffusion coefficient: instead, diffusion displays a broad multicomponent spectrum, indicating that the heterogeneity of surface diffusion fails to average out even over these long times and over distances (similar to 600 nm, the diameter of a diffraction-limited spot) greatly exceeding the size of the polymer molecules. This holds generally when we vary the molecular weight, the surface roughness, and the temperature. It quantifies the long-believed scenario that strongly adsorbed polymer layers (monomer-surface interaction of more than 1kBT) intrinsically present diverse surface conformations that present heterogeneous environments to one another as they diffuse. Bearing in mind that in spite of adsorption from dilute solution the interfacial polymer concentration is high, ramifications of these findings are relevant to the interfacial mobility of polymer glasses, melts, and nanocomposites.