Today I was playing around with several flashes and my Sekonic lightmeter L-358 and a question popped out. First I set my camera at ISO 200, f/8 and 1/60. Then I set my main light at 45 degree towards the subject and set its power until the flashmeter read f/8. Then I set the fill light. I adjusted its power until the Sekonic read f/5.6 (1 f/stop lower than the main light). After setting both lights, I took a reading of the two lights combined and the Sekonic read f/8 plus 7/10 (approximately that is f/10). To get a real f/8 with a shot of the two flashes combined, I needed to dial down the power of the two lights until I got an f/8 on the Sekonic. I think this is normal, for I have a DVD by Sekonic where there is a photographer stating the same effect.
However, I do not really understand why shooting two lights (one of which is underpowered) gives me more power than the main light. I would appreciate if anyone could explain that to me.