i would say that it is very unlikley for one to remember one and not the other. i mean sure you have an orgasm when you ejaculate (usually) which may make it more memorable but still i find it very hard to believe that he can remember one but not the other. also you have to keep in mind that it is possible for pre-ejaculate (pre-cum) to contain sperm cells depending on the last time the guy ejaculated and how well he cleaned up afterwards and if he urinated.... so you cant dispell anything and i reccoment seeing your doctor. they can do a blood test which is more accureate than an at home urin test. also did you follow the directions exactly? i believe it says that the best time to take a test is first thing when you wake up un the morning as that will be when the pregnancy hormones will be most concentrated in the urin....
i hope this helps. let me know if you have any other questions or concerns