I'd guess you aren't playing a fighter-in which case you might notice that the three with much lower THACOs are all fighters of some sort and the three with higher THACOs aren't. Fighter classes get a bonus to THACO every other level (I'm guessing here), more frequently than every other class. Cleric classes get the bonus more often than rogues who get it more often than mages, who hardly ever get the bonus.
Also what Firelighter said about adding skill points to dual-wielding is important(the penalties are -4/-8 with no points, and are reduced by 2 for each point spent down to -0/-2 if you use the maximum three-and the last point won't give you +2 with your main hand, it just reduces the off-hand penalty by another 2).
Weapon proficiency reduces it further (-2 penalty with no points, no penalty with one weapon skill point, +1 with two points, +2 with three and after that I can't tell you since I use the Grand mastery mod and don't know where it differs from core game), and since fighters are the only classes that can put more than 2 points into a weapon skill your fighters are going to get more than +1 this way.
I don't understand how you can main hand a +3 longsword, off hand a +4 longsword and have a mainhand/offhand THACOs difference of 2, but that isn't important.
Assuming (again) that you aren't playing a fighter class your THACOs are right where they should be for early TOB.
Congrats of getting Aerie pregnant. She's a little girl. I hope you're proud.
