According to this 2003 steel warrior thread the off hand shouldn't be anywhere near as good aggro as primary (on a per swing basis)
https://web.archive.org/web/20110104...ead.php?t=3789
According to this a swing from an 8/20 weapon in off hand is worth less than half of an 8/20 swing from primary... that suggests dmg+db we have here for primary is correct, but dmg+db for offhand is erronious.... at least at 2003 era!
Perhaps the 2xdmg+ bonus is me misremembering or remembering a different era. I may have been thinking of the steamfont / meldrath expansion time. Certainly 2003 era posts don't support 2xdmg+db