I don't want to put your builder down as he sounds like a genuine guy but it does sound like to me that a few of your joints are hollow. A quick way to check this without doing any damage is to poke a meat skewer or knitting needle through one or two of the cracked joints. If you meet no resistance when you tap through the outer crust, then it means there is still a void which could cause a weakness to the edge of your slabs leading to cracked jointing or even rocking slabs.......
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