As for the reasons of the upgrading, see that with Starport A (+6 to TL) and Government 5 (+1 to TL), 8 would be the minimum TL that could be rolled for it. SO I guess the initial TL 5 was an errata...

You make a compelling point.
However, as soon as you dip into the "oh, there's an errata, fix it" mindset, I would have thought that switching the starport type between Dentus (UWP C979500 A S) (LBB S3, p20) and Kinorb (UWP A663659 5) so that Dentus gets the type A starport and the Scout Base @ TL=10 while Kinorb gets the Type C starport @ TL=5 would actually make more sense.
Revised Dentus UWP:
A979500
TL DMs = +6 (starport) . +0 (size) . +0 (atmosphere) . +1 (hydrographics) . +1 (population) . +1 (government) = +9DM
1D+9 = 10 is possible (LBB3.81 rules)
Revised Kinorb UWP:
C663659
TL DMs = +2 (starport) . +0 (size) . +0 (atmosphere) . +0 (hydrographics) . +0 (population) . +1 (government) = +3DM
1D+3DM = 5 is possible (LBB3.81 rules)
Still doesn't explain why Regina (UWP A788899 A 2) got a +2TL errata bump from TL=10 to TL=12.
Regina UWP: A788899
TL DMs = +6 (starport) . +0 (size) . +0 (atmosphere) . +0 (hydrographics) . +0 (population) . +0 (government) = +6DM
1D+6DM = 10 is possible (LBB3.81 rules), as is 12 of course, but I'm not seeing any kind of forcibly imposed minimum of 12 here from the +DMs
I've also cross-checked with LBB2.77, p9 to see if there are any changes with respect to the TL DMs in world generation and found none. Continuity of the TL DMs chart is sustained through TTB, p85 as well, so all CT world generation sources are basically in agreement.