Measurement of a photometry refers always to a punctual light source. The light distribution curve is also displayed in evo with a punctual origin.

But for light calculation the complete size of the light output is considered. Therefor the photometry is divide into many sequentes all over the area of the light output. This way the light can visualized over the complete length of the luminaire and refers not just to the point the photometry was measured.

This behavior can easily seen by placing a linear shaped luminaire near a wall.