It is also worth consider that, in the ASC, the term "Unit" may reference "sub-force" rather than the individual model(s). Force Organization is key, having recently re-read Chaos Campaign and the Chaos Campaign: Succession Wars PDFs for reference (for playing TotalWarfare).
If there's just 12 battlemechs for both forces, and if the Mercenaries organize a "unit" by Lances (4) rather than Level IIs (6) the values will shift.
12,000 SP x 1.25 Veteran = 15,000;
. /4 = 3,750 x 0.5 Mercenaries = 1,875 starting SPs
. /6 = 2,500 x 1.5 ComStar/WoB = 3,750 starting SPs
This way is certainly closer to half as previously observed, and equal if the Merks are backed by a major house (underlined, as in my last comment). ...There's a lot to consider from all available sources, so I don't mean to make it sound confusing or more complicated than necessary. Hope this helps.