At a guess i'd say that weight lifting in a gym isolates specific muscles. When youre rolling on the mat its different. Your muscles are working together to give practical strength. Technique and leverage also play their part.
^ Basically this. Isolated muscle training doesn't fully transfer to application. You need inter-muscular training for practical strength. Then, muscle strength is only the potential strength, technique exploits this potential. The better the technique, the more you can use of your potential muscle strength. And finally, on the mat you use different muscles (in relation) than in the gym. That's where americanfighter is right. Core strength for example plays a big role.