Q about dwarfpool, worker Calc Mh/s always higher than sent Mh/s?

Is this a problem? Why is that?

Hashrate

Calculated hashrate based on sent shares during last 30 minutes. After mining start first 15 minutes speed will be shown as "calc...", don't worry if you see shares in column "Last share" If your last share older then 5 minutes, "last share" time will be red.