Yes Mart its sort of right as you see it. If you draw a circle lets say 20mm diameter. Now draw a lobe on there. The "flanks" or ramps of the lobe will actually cover part of your circle beyond the diameter line of that circle-Yes?. In this example your base circle is the 20mm you drew before adding a lobe. When you measure the "width" of the lobe where you expect it to be the base circle diameter, its actually fatter there than the 20mm diameter.
Now if you draw 2 circles, the 20mm one as before and the cam journal in the head lets say 18mm. They have the same centre but the cam circle is 1mm below it (and 1mm above it to make up the 2mm difference).
To measure the true base circle diameter we need to put a straight edge across the "heel" of the cam lobes and measure the distance down to the cam bearing surface, in this case it will be 1mm (its the radius difference of the two circles). We know the cam bearing is 18mm diameter (radius of 9mm) and we have 1mm MORE radius on the heel of the cam. So the diameter of the base circle is 20mm (radius of 10mm). 99% of cams will measure wider than the actual base circle.
Hope thats clearer
I'll try and find some online pics it'll be easier to see I think rather than me waffling on lol
Tony