interesting battery/starting tech question!

[feroskastroke]

2007 F 250, had the servicing done at the dealership during all required intervals and have 82,000 on the truck. i was having problems with the truck cranking over but not starting. The truck would start fine for a week or two at a time and then out of the blue crank but no start. the crank of the engine seemed powerful enough to me to be able to get the required compression for starting. I checked the batteries volts and was recieving the proper 12.7-13 volts for each battery. i took the truck to the dealer for a regular scheduled oil change and multi point inspection. ( this is done for warranty purposes because the engine is under warranty for 100,000) i specifically told them to do a load test on the batteries to check for any broken connections among the cells. they wrote down that the batteries were putting out the factory specs of 750 cca. 6 days later experienced the same symptom of cranking but not starting. i scheduled a diagnostic with another dealership fearing that maybe my Injection control point sensor was going bad or something. after having the truck for a half a day they told me all i needed was 2 new batteries. one only giving off 450cca the other 0cca. i took the truck to autozone and had them test the batteries for a second opinion and sure enough the batteries were bad. i replace both, i understand the first dealership that i took the truck too was not doing a proper multi point inspection or battery diagnostic. MY QUESTION is how can the batteries be bad and still crank the engine but not start it. and only display the sympton every couple weeks. The guy at auto zone explained something about the factory security systems or safety features wont allow the vehicle to start if the batteries were not putting off enough cca's but it would crank over. any ideals on why the vehicle would crank but not start with 2 bad batteries. Tech at 2nd dealership said there were no error codes stored.