Well first then let me quickly apologise for my scepticism as to the validity of your query, a lot of damaging nonsense has been perpetrated on forums in the past by brands envious of the successes of a small company (relative to the corporate giants) like Slingshot.
As to why there are differences, likely the kite speed of the ten permits a shallower depth foil and the twelve is deeper, therefor the centre strut on the larger kite curves more, the shot on the UK forum of them laid flat illustrates a more even difference in flat size, again not that this proves anything. If both you and your girlfriend go out in similar conditions with the 10 & the 12 then the design parameters have been fulfilled.
At 67 kgs in 15 -20 I'd expect to use a 10 whilst a pal at 85 kgs would use a 12 and we should both perform and expect similar performances from the kite.
The key feature of the RPM that separates it from the herd so to speak is it's turning arc and ability to generate Fuel like power boosts in loops as the bridle leaves the wing tip open rather than closed and stalled as most SLE and Bow kites do meaning they pivot on a wingtip. To make this happen the bridle and wing tip in relation to the leading edge take off point have to be worked just right, so a degree of effort would have gone into making them as close as possible so they both generate equal amounts of power, so no matter wether the rider were 67 or 87 kgs the sensation and boost would be the same. That would have been the goal anyway.
Actual canopy size of the finished items would have been a secondary issue as long as the both have the same power range for the given weight of the rider, i.e both you and your girlfriend can stay out as long as each other in the same wind, then they will have achieved their goal.
Now, none of the above is fact, just my speculation and accumulation of experience of having dealt with them these past ten years, now and again a kite size falls weird, I recall a 10.5 model once can't actually remember now what kite it was, but it just came out that size to achieve what they needed for the rider weight range.
I can assure you, they are a straight shooting company, there is no intention to deceive or defraud and the kites you have purchased are the very best at what they do currently on sale, so no need to be concerned.
Communication however is not the companies strongest point, so I hope I've answered your query, there may or may not be a 13 in future that fits in the gap between the 12 and 14 like they did for the 8 and 10 this year, that way folk who like odd sizing can be catered for, at this time I don't know, they are mid way through the launch of the Z kite and the RPM will not launch til spring next season, the 14 also needs some work it's not the fastest kite in the line.