Random Number Generation : same C++ code, two different behaviors

My colleague and I are working on a Monte Carlo project together, in C++. She uses Visual Studio, I use Xcode, we shared the code through git. We are computing American option prices thanks to a given method requiring random number generation. We realized we were getting wrong results for a certain parameter K (the higher the parameter, the more wrong the answer), and my colleague found that changing the random source for Mersenne Twister to rand() (though poor generator) made the results good for the whole range of K.

But when I changed the source on my version of the code, it did nothing.

More puzzling for me, I created a new Xcode project, copied inside it her whole source and it still gives me wrong results (while she gets good ones). So it can't stem from the code itself. I cleaned the project, relaunched Xcode, even restarted my computer (...), but nothing changes : our projects behaves consistently but differently, with the same code behind.

Do you have any idea of what the cause of this dual behavior could be ?