It's hard to know where to begin this answer:
Many, possibly a significant number of males, could benefit from a better understanding of human anatomy and physiology. There are thousands of viable resources on this topic, including numerous approaches to male and female genital A&P, sexual response, etc.
Most women require some form of clitoral stimulation to reach orgasm. That stimulation can come from a finger, toy, cock, lips, or tongue, etc. A well-endowed male, especially a male who is sexually skilled at pleasuring a woman, can produce an orgasm in some women without direct clitoral stimulation. Girth is key to this, and things like body position, fitness, etc. help.
Outside the realm of physiology and skill, the mental/emotional components can also be a factor. Sexual response is at least a 50/50 mind-body balance. If your GF orgasms from her lover, a part of that response can be limerence, attraction, or stronger feelings.