Something else that needs to be taken into consideration is the length of history different car cultures: Europe and the US, 100 years in many cases; Japan and Asia, 50 years or so. With most countries fielding 3 -4 major manufacturers, the numbers game becomes clearer (just to be clear, my point isn't about the ratio of European Fords to US Fords, rather which US models have been recently released in DLC) with Germany, Italy, France, Sweden, and the UK alone representing 15 to 20 makes easily, some with centennial histories, versus the US with 109 year old FoMoCo represented by many Fords one Mercury and no Lincolns (which helps explain the seeming overabundance of Fords); 100 year old GM with Chevy, Pontiac, Cadillac, acouple of Buicks, Oldsmobiles, GMCs and Saturns plus one Hummer; the comparatively young 87 year old Chrysler group comprised of Dodge, Chrysler, Plymouth plus a couple of Jeep and Eagle products; a smattering of single model makes which includes one AMC model plus the Japanese US only brands, and, when taken altogether might make up one additional major manufacturer that makes the big 3 the big 4. That Japan, with it's shorter history and 6 major marques of Toyota, Nissan, Mazda, Honda, Mitsubishi and Subaru has as many cars in the game as it does says that T10 is being pretty even-handed.
I should also point out that in addition to there being no Lincolns and only one Mercury, all European, Australian, and UK Fords are contained in the Ford category whereas GM, in addition to it's many domestic subsidiaries gets Holden, Opel and Vauxhall to belie its monolithic presence in the game - of course it looks like there're too many Fords when arranged in this manner. I haven't gone through and added everybody's cars up, but FoMoCo may actually be underrepresented in Forza.