VNAV and SPEED lights illuminated simultaneously

During my latest flight I noticed the following: at a certain point before level off during a VNAV climb the SPEED light comes on. I get the impression that as the aircraft is climbing rapidly, at one point N1 is replaced by FMC SPD in the A/T logic and at that point the SPEED light comes on.Now, it seems to me that you could never have the VNAV and the SPEED light on at the same time. Boeing logic is always that a button on the MCP with a light on means that that particular mode can be deselected by pushing that button again. Obviously, while VNAV is in control, you couldn't deselect the A/T mode, and indeed, in the situation I describe above pushing the lighted SPEED button has no results.Leo Bakker