Ct scan showed .7mm cyst on pineal gland. another ct a few weeks later showed no abnormalities? how is this possible?

It depends on the resolution of the CT and if they were both read by the same person. Discuss with your Drs. Remember how small 0.7 mm is. The lead in a #2 pencil is 10.3 mm in diameter, then 0.7 mm is 14 times smaller than #2 lead. If head is shot from a slightly different angle, then it may be missed. Ask to be shown by your Dr on the X-ray so you can see. Check if it is 0.7 mm or 0.7 cm.