So why if you read study's from cardiac mri's the normal values fluctuate so much? for instance one study said normal rv-edv:150ml average and another study said average 260ml. has this something to do with imaging techniques or with calculations?

Yes: It could be variations in the imaging technique, the accuracy of the MRI type, the protocol used, the cooperation or fluid status of the patient, the software used or formulas used to calculate the volumes, the preparation of the images, or the interpreter doing the MRI reading among several other things. This is why the results must be taken into context with the reason for ordering the test.