Sekonic L-358 studio light reading

I have a Sekonic L-358 flash meter and using it with 400W studio lights. When I measure the light using Cord Flash mode (s 1/60), the analog scale shows letter U, which means "below display range". I tried various shutter speed measurements, but the letter U is still there.

I don't understand what was "below display range". Is it the shutter speed, or aperture, or exposure?

Another question: I have purchased a set of Strobes from a shop. They are the same brand and same 400W strobes. But one of them outputs 1 stop less light than the other. I have measured the light using my light meter. Also when I take a shot of flat copies such as painting, the difference is obvious from the images taken. Is it normal for the strobes to output different amount of light even though they came as a set?